Effects of sulfur compounds and CO molecule on the catalytic performance of the reduced and sulfided NiMo/Al2O3 catalysts in hydrogenation of linear aldehydes (hexanal and propanal) are studied. Although the steady-state reaction experiments showed a decrease in activity and selectivity of reduced catalysts when a model sulfur compound was introduced into the feed, the reaction performance of the reduced catalyst still remained superior to that of the sulfided catalysts. The studies with XPS, TPD, and volumetric chemisorption measurement techniques shown that, there is partial sulfidation of the reduced catalysts when exposed to sulfur under reaction conditions, however, it does not convert the catalyst to a molybdenum sulfide phase. Instead oxygen-sulfur exchange on the surface leaves behind oxisulfide species, with catalytic activity closely resembling that of the reduced catalysts. The presence of CO in the feed mixture leads to a decrease in hydrogenation activity and selectivity for both reduced and sulfided catalysts, due to the competitive adsorption between aldehyde and CO, although the effect of CO is completely reversible. (c) 2005 Elsevier B.V. All rights reserved.