This research was focused at getting deeper insight into the circumstances of "cobalt aluminate'' formation in alumina-supported cobalt catalysts. It was desired to determine which state(s) of cobalt (i.e. reduced or oxidized) and what reaction conditions play a significant role in the aluminate formation at the relatively mild conditions used for reduction or Fischer - Tropsch synthesis. Reduction and hydrothermal pretreatments were applied to influence the formation of nonreducible Co-support compounds. Temperature-programmed reduction (TPR) was used to measure the amount of reducible cobalt compounds formed in 20 wt% Co/Al2O3 catalyst. The results reveal that the presence of partially reduced cobalt in a well-dispersed form is required for the formation of cobalt aluminate. The presence of water vapor promotes cobalt aluminate formation probably by hydrating the alumina and possibly by partially reoxidizing highly dispersed cobalt.