Reducibility of Co species in silica supported Fischer-Tropsch catalysts was studied using in situ XRD, in situ EXAFS, and FTIR spectroscopy with carbon monoxide as a molecular probe. Crystalline Co phases in the oxidised catalysts were characterised using XRD. In the oxidised samples with a large concentration of amorphous phase, EXAFS showed the presence of small oxide clusters including several Co atoms. It was found (in situ XRD, EXAFS) that calcination of oxidised Co catalysts under inert atmosphere resulted in a selective transformation of Co3O4 to CoO at 623-673 K. FTIR spectroscopy with CO as a molecular probe revealed the presence of different sites associated with Co after the reduction of the catalysts with hydrogen at 723 K : Co metal sites (v(CO) = 2025 cm(-1)), Co2+ ions in the crystalline phase of CoO (v(CO) = 2143 cm(-1)), and Con+ species in the amorphous phase (v(CO) = 2181 cm(-1)). The results (XRD, EXAFS, FTIR) showed that the hydrogen reduction properties of particles of cobalt oxide on silica depended on the size of the Co3O4 crystallites. The ease of reduction to metal species decreased from larger (200-700 Angstrom) to smaller (60 Angstrom) particles.