In order to assess the distribution and the promotional effects of La 3+ , a characterization study has been performed on a series of La 3+ -promoted 20 wt% Co/SiO 2 catalysts of varying La : Co atomic ratios (0, 0.05, 0.1, 0.2, 0.3, 0.5, and 0.75) prepared by impregnation of prereduced and passivated Co/SiO 2 in order to minimize effects of promotion on Co particle size distribution. La 3+ was found to be well dispersed over the Co/SiO 2 catalyst. However, it did not appear to be evenly distributed on the Co metal surface. From temperature programmed reduction, low La 3+ loadings (La/Co≤0.2) were found to moderate the strong Co–support interactions resulting from aqueous impregnation of Co/SiO 2 and to improve the reducibility of the Co oxide. [Aqueous impregnation of prereduced Co/SiO 2 has been shown earlier to result in the formation of Co silicates (Haddad, G. J., and Goodwin, J. G., Jr.,J. Catal. 157, 25 (1995))]. On the other hand, high La 3+ loadings (La/Co≥0.5) appeared to induce La–Co compound and/or nonreducible Co-silicate formation, possibly due to the higher pH of the impregnating solution. The results indicate the multiple effects that promoters may have on supported metal catalysts. In addition to possibly affecting reaction on the sites, they may act to modify significantly the structure of the metal catalyst, for example by moderating metal–support interactions. Obviously, one expects that promoted catalysts prepared directly from precursors may exhibit large differences in metal dispersion. However, even when promoted catalysts are prepared from a precalcined or prereduced supported metal catalyst, the act of adding a promoter may have as much an effect in modifying the catalyst as the final presence of the promoter.