The effect of the nature of the lanthanide (Ln = La, Ce, Pr, Sm, Gd) on the structure and reactivity of Co/SiO2 catalysts for CO hydrogenation (i.e., Fischer Trospch synthesis) was investigated. In-situ temperature programmed reduction with extended x-ray absorption fine structure and x-ray absorption near edge spectroscopy (TPR-EXAFS/XANES) of the structure of both Co and Ln containing phases under activation and CO hydrogenation conditions were performed. Concerning catalyst selectivity (made at comparable conversion levels), while methane selectivity was higher for the Gd-doped catalyst (similar to 100%, relative) compared to the unpromoted catalyst, the selectivity to olefins plus alcohols and C-2-C-4 products was higher (similar to 35%, relative), compared to the unpromoted catalyst. The Ce-promoted Co/SiO2 catalyst presented the highest oxygenate/olefin selectivity 40%), among the promoted catalysts tested at a similar conversion level of similar to 20%. Under reactive conditions (both following H-2 activation and during CO + H-2 flow), a mixture containing small LnO(x) /CoO/Co degrees nanocrystallites likely constitute the active sites during reaction. These results imply that the presence of the lanthanide likely introduces surface defects in the oxides (LnO(x) and/or CoO) located at the metallic cobalt nanoparticle rim which may serve as active sites for active O-containing species (e.g., mobile Type II OH groups) that may either serve as chain termination species, or generate chain terminating species such as formates (i.e., essentially molecularly adsorbed CO) upon CO adsorption.