The work presents the effect of K-doping on V/Ti-oxides taking into account: the surface acid–base properties and the structure of surface vanadia species in respect to the catalyst performance and deactivation. The structure of active surface species determines redox properties, which are related to the catalytic performance by the Mars–van Krevelen mechanism. The reducibility of surface vanadia is studied by temperature-programmed reduction (TPR) in H2. The molecular structure of surface vanadia is determined by FT-Raman spectroscopy in a controlled atmosphere. Surface acid–base properties are characterised via temperature-programmed desorption (TPD) of pyridine with mass spectrometric analysis of the products. Transient response techniques with continuous monitoring of the composition of gaseous phase are applied to follow the catalyst surface transformations. Evolution of benzaldehyde (BA) formed during interaction of toluene with the pre-oxidised catalyst (without gaseous oxygen) gives information about the nucleophilicity of surface oxygen. Addition of potassium to surface vanadia leads to an increased oxygen nucleophilicity, resulting in a higher selectivity towards BA formation. In general, increase in surface basicity decreases catalytic activity, but at the same time the catalyst deactivation due to coking is suppressed. This allows catalyst optimisation in view of a better control of the partial oxidation process.