The role of water in the deactivation of Al2O3-supported cobalt Fischer-Tropsch catalysts has been studied at different water partial pressures. In addition, model studies at nonreacting conditions using H2O/H-2 feeds were carried out in order to study the possible reoxidation of cobalt by water. High-pressure gravimetry, temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) were used as characterization methods in the model studies. The kinetic studies showed significant deactivation when water was added to the feed in fixed-bed reactor experiments. In the model studies, a large extent of bulk cobalt reoxidation was only observed in the absence of H-2. Only a small amount of reoxidation could be observed when H-2 was present, even at high H2O/H-2 ratios. However, XPS studies indicated significant surface oxidation of cobalt at a lower H2O/H-2 ratio. Surface oxidation or oxidation of highly dispersed cobalt phases is therefore concluded to be responsible for the observed deactivation. The possibility of changes in the phase distribution of the catalysts caused by the presence of high water partial pressures is discussed on the basis of TPR studies of H2O/H-2- and H2O/He-treated catalysts.