Steam reforming of acetic acid over Pt/ZrO2 catalysts has been investigated. Pt/ZrO2 catalysts are very active, completely converting acetic acid, and give a hydrogen yield close to thermodynamic equilibrium. The catalyst deactivated by formation of oligomers which block the active sites. The mechanism of both the reaction and the deactivation was studied with kinetic and spectroscopic measurements on Pt/ZrO2, ZrO2, and Pt black. It is shown that Pt is essential for steam reforming to proceed. ZrO2 is needed to activate steam. ZrO2, on the other hand, is also active for oligomer¿precursor formation under the conditions investigated. Results obtained in the study show that steam reforming takes place at the PtZrO2 boundary and that deactivation occurs when this boundary is blocked by oligomers.