The deactivation of Co/SiO2 catalyst for Fischer-Tropsch synthesis (FTS) at different H-2/CO ratios was investigated by XRD, FTIR, BET, XPS, TPR and H-2 chemisorption. It was found that the deactivation rate of the catalyst increased with the rise of the H-2/CO ratio. The generation of silicates and/or hydrosilicates species was evidenced by TPR and XPS, and their amounts were monotonously enhanced with increasing H-2/CO ratio, which suggested that the deactivation was caused by the transformation of metallic cobalt into inactive silicates and the high partial pressure of H-2 facilitated the formation of the silicates. Moreover, the percentage loss of the surface cobalt was larger than that of bulk cobalt, suggesting that the cobalt silicates and/or hydrosilicates species were formed mainly on the surface of the catalyst or in the small crystallites. For the catalyst run at H-2/CO ratio of 1, it was observed that the sintering also contributed to the catalyst deactivation, but it was a less important factor for the deactivation. (C) 2006 Elsevier B.V. All rights reserved.