Rh-silica catalysts having the same physical parameters despite their different Rh contents were prepared using water-in-oil (w/o) microemulsions, and the effects of Rh content on the catalytic behavior were investigated in the hydrogenation of carbon monoxide. It was found that both the product selectivity and the turnover frequency changed dramatically with the Rh content. The selectivity to C2+ oxygenated compounds increased with increasing Rh content, while the selectivity to hydrocarbons and the turnover frequency decreased.