Co/MCM-41 catalysts were prepared using the incipient wetness impregnation technique with aqueous solutions of different cobalt compounds such as cobalt nitrate, cobalt chloride, cobalt acetate, and cobalt acetylacetonate. MCM-41 is known to have a restricted pore structure; however, using organic precursors such as cobalt acetate and cobalt acetylacetonate resulted in very small cobalt oxide particles that could not be detected by XRD even for a cobalt loading as high as 8 wt%. These cobalt particles were small enough to fit into the pores of MCM-41. However, they were found to chemisorb CO in only relatively small amounts and to have low activities for CO hydrogenation -probably due to the formation of cobalt silicates. The use of cobalt chloride resulted in very large cobalt particles/clusters and/or residual Cl--blocking active sites, and, consequently, very small active surface area was measurable. The use of cobalt nitrate resulted in a number of small cobalt particles dispersed throughout MCM-41 and some larger particles located on the external surface of MCM-41. Cobalt nitrate appeared to be the best precursor for preparing high-activity MCM-41-supported cobalt Fischer - Tropsch synthesis catalysts.