The hydrodeoxygenation (HDO) network of aliphatic esters was investigated over reduced and sulfided NiMo/gamma-Al2O3 catalysts in a batch reactor at 250 degrees C and 7.5 MPa. Methyl and ethyl heptanoate and their main HDO reaction intermediates were used as reactants. The participation of surface OH- and SH- groups in the various reaction steps and the stability of the surface sulfur are discussed. Although the surfaces of the reduced and sulfided catalysts under H-2 have identical properties, the reactivity of the oxygen-containing compounds is greater over the sulfided catalyst. This is due to the stronger nucleophilic strength of the SH- groups compared to the OH- groups. Methyl and ethyl heptanoate react via various reaction pathways, of which alkaline hydrolysis is demonstrated to be of special importance. Sulfur-containing intermediates are formed in the HDO reactions with surface SH- groups. Accordingly, sulfur level on the catalyst surface is diminished. (C) 2009 Elsevier B.V. All rights reserved.