A series of Co-Ru-Zr/SiO2 catalysts were prepared by the co-impregnation method in order to understand the effects of calcination temperature on their activity and stability. The crystallite size of cobalt obtained from H-2 chemisorption decreased up to a calcination temperature of 300 A degrees C and then increased to reach a reasonably constant value. The activity for CO hydrogenation also increased upto this temperature and then decreased to be stabilized finally at a value smaller than the optimum one. The turn-over frequency values are not significantly altered on the catalysts calcined above 220 A degrees C. It was observed that in the presence of chlorine released during the decomposition of the zirconium oxychloride precursor, the dispersion of cobalt crystallites was altered due to the different environment of chlorine and water evolved during the reduction step.