Co/gamma-Al2O3 catalysts were prepared by the slurry impregnation of an aqueous solution of cobalt(II) nitrate precursor. Nitric acid or ammonium hydroxide was added to the cobalt nitrate solution, during impregnation, to give an acidic or basic environment. The changes in the particle size of cobalt species were estimated by X-ray diffraction (XRD) and hydrogen chemisorption. The reduction degree of cobalt oxides was measured by temperature-programmed reduction (TPR). The catalysts prepared under acidic conditions showed a higher reduction degree compared to those prepared at higher pH because of the reduced salt-support interaction. During the Fischer-Tropsch synthesis at 220 degrees C, employing the catalysts prepared at a different pH (0.80, 4.94, 9.96, and 11.12), a considerable difference in the initial activity was observed, depending upon the cobalt metal surface area. However, after stabilization, all of the catalysts attained a similar level of conversion, possibly because of the active-site rearrangement, deactivation, and wax formation on the catalyst surface. At a higher reaction temperature of 240 degrees C, the catalysts prepared at lower solution pH exhibited higher conversion than those prepared at higher solution pH. The cobalt species on the catalysts prepared under acidic conditions had a heterogeneous particle size distribution, showing higher steady-state activity, because of the reduced interaction with the support. The product distribution revealed a higher selectivity to C-1 and C8+ on the catalyst prepared with a higher solution pH.