Mechanistic studies on steam reforming of acetic acid over Pt/ZrO2 catalysts were performed as extension of our previous work [K. Takanabe, K. Aika, K. Seshan, L. Lefferts, J. Catal. 227 (2004) 101]. An overall picture of the bifunctional mechanism is established for steam reforming of acetic acid, where both Pt and ZrO2 participate in the reaction. On Pt, bond breaking of acetic acid proceeds to form H-2, CO, CH4, and CO2 into gas phase, and to form the carbonaceous residue (most probably CHx species), which can block Pt surface. Both pulse experiments as well as in situ IR data demonstrate that H2O can be activated on ZrO2 to create supplementary surface hydroxyl groups, which react to gasify the residue on Pt to give chiefly steam-reforming/water-gas shift products (H-2, CO2), and thus the catalytic cycles continue. Importance of the Pt-ZrO2 boundary sites is confirmed by the fact that the removal of the carbonaceous residue situated at the boundary sites results in catalyst regeneration. (c) 2006 Elsevier Inc. All rights reserved.