The decomposition of the Co4-nRunHn(CO)xSiO2 (where n = 0–4 and x = 12 or 13) and (CO4(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 Co4(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°C for (Co4(CO)12 + Ru4H4(CO)12), 127°C for CoRu3H3(CO)12, 133°C for Co2Ru2(CO)13, 137°C for Co4(CO)12, 144°C for Co2Ru2H2(CO)12, 148°C for Co3RuH(CO)12 and 185°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.