Reduction of unpromoted Co- and promoted Co-ThO2-MgO-Kieselguhr catalysts has been studied with X-ray photoelectron spectroscopy and temperature-programmed reduction techniques. A mechanism of reduction is proposed based on spectroscopic measurements of the surface phases before and after H2 treatment at 400 °C. Cobalt oxide Co3O4 is present on all air-calcined catalysts, and is reduced rapidly to CoO at temperatures >300 °C in hydrogen. CoO is then more slowly reduced to Co metal on the unpromoted catalysts at 400 °C. If the promoters MgO and ThO2 are present, a reaction between CoO and promoter can occur resulting in new phases which are more resistant to reduction than CoO alone. XPS results suggest one phase as a CoO-MgO solid solution, but the CoO-ThO2 phase could not be identified. The cobalt in magnesia-promoted CoO phases is reduced completely to the metal at much higher temperatures (500-700°C) but at 400°C the surface reduction is limited to 55% ± 5 after 1 h. Variability in the degree of reduction of various promoted cobalt catalysts is therefore dependent on the nature and loading of the promoter and the reduction temperature. Promotion of a Co-Kieselguhr catalyst with ThO2 or MgO does allow a higher degree of control over the CoO reduction kinetics than is found with unpromoted catalysts.