The decomposition of the Co4-nRunHn(CO)(x)/SiO2 (where n = 0-4 and x = 12 or 13) and (Co-4(CO)(12) + Ru4H4(CO)(12))/SiO2 catalysts was studied by temperature-programmed reduction (TPR) and in-situ diffuse reflectance FT-IR techniques. The FT-IR studies suggested that Ru4H4(CO)(12), CoRu3H3(CO)(12) and Co3RuH(CO)(12) clusters on silica were more stable in room temperature than Co-4(CO)(12), Co2Ru2H2(CO)(12) and Co2Ru2(CO)(13) clusters. The FT-IR measurements also indicated that all precursors decomposed during thermal treatment, but the distinct temperature of decarbonylation was not necessarily noticeable. On the other hand, in TPR the CO desorption peak maxima for the precursors were 123 degrees C for (Co-4(CO)(12) + Ru4H4(CO)(12)), 127 degrees C for CoRu3H3(CO)(12), 133 degrees C for Co2Ru2(CO)(13), 137 degrees C for Co-4(CO)(12), 144 degrees C for Co2Ru2H2(CO)(12), 148 degrees C for Co3RuH(CO)(12) and 185 degrees C for Ru4H4(CO)(12). Thus, the catalysts with 1:1 atomic ratio of Co:Ru decarbonylated at different temperatures. The temperature of decarbonylation was lower for Co2Ru2(CO)(13)/SiO2 than for Co2Ru2H2(CO)(12)/SiO2, i.e., cobalt influenced the decarbonylation more in the first case. In accordance, the CO hydrogenation activity and selectivity results for the decarbonylated catalysts suggested that Co2Ru2(CO)(13)/SiO2 exhibited the characteristics of cobalt more than Co2Ru2H2(CO)(12)/SiO2 did.