What are the social impacts of a new, less efficient public transport system in a city that is known for its inequality concerning access to daily activities and participation in urban life? More precisely, what are the implications of a re-regulated transport system, if the city has previously gone through a long period of deregulation? Are the changes limited to altered accessibility conditions, or are there further consequences for people's travel competencies, habits and mobility patterns? And what does that mean for the dissolution or manifestation of social inequality and risks of social exclusion? With a specific regard to these interests, this work specifically focuses on the case of Santiago de Chile. There the public transport policy passed from a period of complete de-regulation in the 1970s and 1980s to a period of re-regulation as of the 1990s. During the deregulation period, the services had been characterized by an uncoordinated oversupply of private busses that existed in parallel to the more expensive public metro. In order to suspend the stigmatization of bus-based transport as the 'mode for the poor', the sophisticated 'Transantiago' bus system was implemented in February 2007, which was modelled of the famous BRT system 'Transmilenio' in Bogotá. The Transantiago project included the total re-design of the network system and the private operating companies at one glance. It introduced new vehicles and infrastructures as well as an electronic ticketing system that combined the bus and metro systems in a tariff union. Unfortunately, the ambitious project failed, due to various political, technical and social reasons. Up until today, the acceptance and satisfaction of the inhabitants with Transantiago have been rather low. Being aware of the importance of the technical problems, this study concentrates on the social problems and the related impacts Transantiago has had on people's daily life. The research is done on the basis of three hypotheses that are strongly based on the concept of 'motility', i.e. the potential to be mobile, including the set of factors which enable spatial mobility (access, competencies and appropriation). The hypotheses are tested on the basis of a mixed approach that combines various qualitative methods with an 'ad-hoc' survey of 2000 households in five different areas of the metropolitan Santiago area. The survey results are analysed on the basis of various spatial and statistical procedures, including, among others, structural equation modelling. We first consider the relation between the differences related to mobility and the social differences during the period of deregulation and describe the major mobility patterns and habits developed during that time (hypothesis 1). Then we come to the Transantiago failures and the changes imposed on people's accessibility conditions, travel competencies, preferences and habits (hypothesis 2). Finally we also explore the impact the Transantiago system has had on people's daily life, more precisely on people's daily activities and the activity locations as well as the main travel modes used to reach these locations. Comparing people's current activity patterns and travel patterns with those of the deregulated transport period, we intend to reveal if Transantiago has aggravated or alleviated existing mobility inequalities and related risks of social exclusion (hypothesis 3). The results show that mobility patterns developed during the deregulated period have had a strong negative impact on people's ability to get used to and accept the sophisticated Transantiago system. Particularly people of low income, people of a lower education level, people without access to a private car and people who live in peripheral areas and don't have direct access to the metro were most heavily impacted. Sometimes, older people and women within these groups suffered more than younger adults and men. The initial difficulties have had a negative impact on the current evaluation and image of public transport, and on the other hand they have exacerbated already existing risks of social exclusion and segregation. Especially the more 'vulnerable' inhabitants have adapted their daily activity patterns and travel patterns. They travel less for job-related and leisure-related purposes; they prefer other modes to bus-based public transport and remain often closer to their place of residence than before the Transantiago implementation. Nonetheless, Transantiago has also had some positive impacts such as increased spatial mobility for inhabitants with direct access to the metro, low travel expenditures due to permanent operating subsidies as well as reduced environmental emissions and safety problems. After all, it is hardly possible to definitively determine if the current mobility-related and social inequalities are effectively a consequence of the Transantiago system. It is also unclear if these inequalities have today a more widespread impact than during the deregulated transport period. In general, throughout this study the important influence of income and social status on inhabitants' mobility conditions and travel behaviour is revealed. Alternative concepts related to residential preferences and urban lifestyles turn out to be less pertinent to the explanation of social differences in travel behaviour. This also means that transport policy aimed at equal mobility conditions can only give a contribution to social inclusion. Therefore, additional urban and social measures are needed. This work consists of six parts. Following the introduction (chapter 1), a broad theoretical framework is presented where the different topics of interest are discussed and examples of empirical studies are presented, with a focus on emerging economies (chapter 2): transport regulation and deregulation and recent trends in public transport policy and supply (2.1); the various spatial, social and individual determinants of travel behaviour and mobility patterns and the implications for transport research (2.2); mobility inequality as possible source of social exclusion, which calls for transport policy making with a focus on social equity (2.3). After having presented the research hypotheses and the methodology employed (chapter 3), the case study area of Santiago is introduced. In this context we describe the development from the deregulated transport period up until today with regard to four issues: the spatial and social structure of the city as well as the development of transport policy and people's mobility patterns (chapter 4). Each of the three hypotheses is analysed and evaluated on the basis of the own empiric works (chapter 5). The work concludes with a synthesis as well as some final remarks and implications for the development of transport policy in Santiago and other cities in emerging economies, where comprehensive transport changes are envisaged (chapter 6).