The oxidation of o-xylene over a commercial V2O5/TiO2 (anatase) catalyst was studied in the TAP (Temporal Analysis of Products) reactor system. The transient method with high time resolution has led to new insights into the reaction mechanism. The selective oxidation is shown to consist of a sequence of steps, involving intermediates which desorb as o-toluotaldehyde and phtalide. The key reaction step in the non-selective oxidation is shown to be a decarbonylation of a surface species, leading to a surface benzoate. The kinetics of the elementary steps in the reaction were derived from a quantitative analysis of the transient responses on o-xylene pulses.