Results of the characterization of six Co-based Fischer - Tropsch ( FT) catalysts, with 15% Co loading and supported on SiO2 and Al2O3, are presented. Room temperature X-ray diffraction (XRD), temperature and magnetic field (H) variation of the magnetization (M), and low-temperature (5 K) electron magnetic resonance (EMR) are used for determining the electronic states (Co-0, CoO, Co3O4, Co2+) of cobalt. Performance of these catalysts for FT synthesis is tested at reaction temperature of 240 degreesC and pressure of 20 bars. Under these conditions, 15% Co/SiO2 catalysts yield higher CO and syngas conversions with higher methane selectivity than 15% Co/ Al2O3 catalysts. Conversely the Al2O3 supported catalysts gave much higher selectivity towards olefins than Co/SiO2. These results yield the correlation that the presence of Co3O4 yield higher methane selectivity whereas the presence of Co2+ species yields lower methane selectivity but higher olefin selectivity. The activities and selectivities are found to be stable for 55 h on-stream.