The vapor phase oxidation of propene to acrolein is a highly exothermic reaction. To ensure isothermal reaction conditions, a catalytic wall reactor was used for detailed investigations on the reaction behavior on a multicomponent bismuth -molybdate oxide catalyst. The reaction temperature showed only small influence on the selectivity to acrolein, but a significant optimum at about 360 degreesC is observed. The results further indicate a change in the rate-determining step: while at low temperatures (<360 degreesC) catalyst reoxidation is rate determining, with increasing oxygen content accelerating the formation of acrolein considerably, its influence disappears at higher temperatures. Not only does water increase the selectivity to acrolein, but also, at low temperatures, it improves catalyst reoxidation remarkably. Additionally, the formation of the most important side products (acrylic acid, carbon oxides, acetaldehyde, formaldehyde, and acetic acid) was observed, depending on the reaction parameters.