Microfluidic devices are a promising new tool for studying and optimizing (bio)chemical reactions and analyses. Many (bio)chemical reactions require accurate temperature control, such as for example thermocycling for PCR. Here, a new integrated temperature control system for microfluidic devices is presented, using chemical and physical processes to locally regulate temperature. In demonstration experiments, the evaporation of acetone was used as an endothermic process to cool a microchannel. Additionally, heating of a microchannel was achieved by dissolution of concentrated sulfuric acid in water as an exothermic process. Localization of the contact area of two flows in a microfluidic channel allows control of the position and the magnitude of the thermal effect. © The Royal Society of Chemistry 2003.