A reaction network of aldehyde hydrogenation over NiMoS/Al2O3 catalysts was studied with aldehydes with straight and branched carbon chains and different chain lengths as feed materials. The reactions in the gas phase and the liquid phase were compared. The main reaction in the aldehyde hydrogenation process is the hydrogenation of the C=O double bond, which takes place over the coordinatively unsaturated sites. The major side reactions are self-condensation of aldehydes and condensation of aldehydes with alcohols. Both reactions involve alpha-hydrogen and are primarily catalyzed by acid-base bifunctional sites over the exposed Al2O3 surfaces. (c) 2004 Elsevier Inc. All rights reserved.