To elucidate bio-oil catalytic reforming mechanisms, acetic acid has been selected as a model compound to carry out density functional theory calculations on its decomposition pathways over an active Co stepped surface. The adsorption energies and stable geometries of the reactant and important reaction intermediates have been obtained. Preferential adsorption on a step as opposed to a flat surface has been discerned. The activation barriers and reaction enthalpies for various elementary reactions involved in acetic acid decomposition have also been obtained. Four likely parallel decomposition pathways with activation barriers of less than 0.9 eV have been identified. The most kinetically favorable decomposition pathway is CH3COOH -> CH3CO -> CH2CO -> CH2 -> CH. The rate-determining step is CH3CO dehydrogenation, with a reaction barrier of 0.52 eV. In addition, the formation and primary decomposition pathways of the critical intermediate acetone have also been investigated. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.