In situ XAFS spectroscopy studies have been conducted at reaction conditions on potassium-promoted cobalt catalysts supported on SiO2 and Al2O3, and on similar samples without potassium. Measurements were conducted at temperature up to 200 degrees C under both reducing (flowing H-2) and simulated Fischer-Tropsch (F-T) reaction (flowing CO/H-2) conditions. K and Co K-edge XANES spectra indicate that K interacts with both the Co phase and the SiO2 support, and that K promotion inhibits the reduction of the Co/SiO2 catalyst in H-2 at 200 degrees C. However, under CO hydrogenation conditions in the presence of water vapor, the differences in degree of reducibility are less since the unpromoted catalyst becomes more oxidized. For the K-promoted Co/Al2O3 catalyst, Co K-edge XANES and EXAFS spectra show that Co is initially present as a mixture of Co3O4 and CoO. Under H-2 at 200 degrees C, these oxides are reduced to a mixture of CoO and Co metal. During reaction at 200 degrees C in H-2/CO, the Co phases are reconverted to a mixture of Co3O4 and CoO. Without K, the Co in Co/Al2O3 is reduced almost completely to Co metal under H-2 at 200 degrees C and remains in that form even during reaction at 200 degrees C in H-2/CO. Only a minor trace of CoO is observed.