The oxidation of supported cobalt based slurry bed Fischer-Tropsch catalysts by means of water was studied. Water is one of the Fischer-Tropsch reaction products and can probably cause oxidation and deactivation of a reduced cobalt catalyst. Model experiments using Mossbauer emission spectroscopy and thermogravimetry as well as realistic Fischer-Tropsch synthesis runs were performed. it was demonstrated that Mossbauer emission spectroscopy can successfully be applied to the investigation of high cobalt loading Fischer-Tropsch catalysts. Strong indications were found that oxidation of reduced cobalt catalysts occurs under realistic Fischer-Tropsch conditions. Mossbauer emission spectroscopy and thermogravimetry results showed that the oxidation depends on the P-H2/P-H2O ratio, and that oxidation proceeds to less than complete extents under certain conditions. The formation of both reducible and less reducible cobalt oxide species was observed, and the relative ratio between these species depends on the severity of the oxidation conditions. (C) 2000 Elsevier Science B.V. All rights reserved.