Nanocrystalline cobalt sulfides were prepared by the reactions between cobalt chlorides and sodium polysulfide via a toluene thermal process in the temperature range 120-170 degrees C. Two single phases of Co9S8 and CoS2 were obtained. TEM microphotos showed that the Co9S8 and CoS2 particles were both spherical in shape with sizes of about 20 nm. Chemical analysis gave the formulas Co9S7.93 and CoS1.97, respectively. The transformations among cobalt sulfides (Co9S8, Co3S4, and CoS2) with changing reaction conditions and precursors were studied.