The effects of reaction temperature, feed ratio, space time, and CO percentage in feed on the deactivation conditions of a co-precipitated 36 wt% Co/Al2O3 catalyst in CO hydrogenation were investigated. Environmental-SEM-EDX and temperature-programmed reduction (TPR) studies were performed on used catalysts to investigate the effect of reaction conditions on catalyst deactivation. Intensive coke deposition on the catalyst was observed at a reaction temperature of about 573 K. Increasing the H-2/CO ratio also increased the catalyst activity. Methane formed as the main product at high H-2/CO ratios. Although catalyst activity did not change significantly during the 9-h reaction period at all conditions, a maximum of 6.8% C deposition was observed with increasing H-2/CO, and this caused a 2% decrease in the total surface area of the used catalyst. Space time and CO percentage in the feed had less influence on catalyst decomposition.