In supported cobalt catalysts, the formation of cobalt-support compounds can result in lower activity of the catalysts. It has been found that water vapor present during standard reduction affects the degree of reducibility of cobalt in CoRu/gamma -Al2O3. In this study, the impact of water vapor on the formation of Co-support compounds and the resulting characteristics of Co/gamma -Al2O3 and CoRu/gamma -Al2O3 catalysts were investigated to develop a better understanding of the nature of the Co-support compounds formed and the effect of noble metal promotion on their formation. The Co catalysts were reduced under differential conditions with and without added water vapor and then characterized. Co-support compound formation could not be detected by X-ray diffraction; however, Raman spectroscopy gave useful information about the Co "aluminate" formed. Temperature-programmed reduction indicated that the degree of reduction of the catalyst samples was lower when additional water vapor was introduced during reduction, but to a lesser degree when the Ru promoter was present. The Raman spectroscopic results suggest that the Co aluminate formed is not identical to CoAl2O4 (spinel) but is probably a surface compound deficient in Co. This compound formation is a major cause for differences seen in the degree of reducibility of Co/alumina catalysts after initial reduction, hydrogen chemisorption capacity, and Fischer-Tropsch synthesis activity.