The isomerization of methyl cis-9-octadecenoate (cis isomer) to methyl trans-9-octadecenoate (trans isomer) was studied in the presence of CoSn/ZnO catalysts under hydrogen pressure. The cis-trans isomerization of methyl cis-9-octadecenoate over a CoSn/ZnO catalysts prepared by co-impregnation, was strongly influenced by the nature of the catalytic sites. The support ZnO did not favour this reaction but the hydrogen played an important role since it influenced the relative amount of different cobalt species. In fact cobalt-containing sites (zerovalent species as well as cobalt oxides) predominated over these reactions. The tin species. which acted as a cobalt modifier for the preferential adsorption of the carbonyl group, inhibited the isomerization of the olefinic bond. Such changes could have also been due to a significant diminution of the cobalt content at the catalyst surface especially for a bulk Sn/Co > 1. Consequently, the active sites for the cis-trans isomerization reaction should be cobalt species. (C) 2009 Elsevier B.V. All rights reserved.