The choice of a support or promoter may influence the activity and selectivity of a Fischer-Tropsch catalyst. In this study the effect of modification of an alumina support with molybdenum on the performance of Co/gamma-Al2O3-catalysts is investigated. The addition of molybdenum leads to a strong decrease in the available metal surface area and a higher acidity in the reduced catalyst. Carbon monoxide can adsorb both on molybdenum sites within the catalysts and on metallic sites. The strength of adsorption of CO on metallic sites increases due to the presence of molybdenum in the catalyst. The initial activity in the Fischer-Tropsch synthesis per unit metal surface area does not change much, but molybdenum-modified catalysts deactivate faster. The selectivity of molybdenum-modified Co/gamma-Al2O3 catalysts during the Fischer-Tropsch synthesis is strongly affected by the presence of the acid sites within the catalyst. The enhanced extent of double bond isomerisation on the acidic sites results in a decrease in the chain growth probability, and in an increase in the amount of branched compounds formed in the Fischer-Tropsch synthesis over metallic cobalt sites. (C) 2007 Elsevier B.V. All rights reserved.