Formation of CdS nanocrystals in the classic approach (with octadecene (ODE) as the solvent and elemental sulfur and cadmium carboxylate as the precursors) was found to be kinetically dependent on reduction of elemental sulfur by ODE, which possessed a critical temperature (similar to 180 degrees C). After elemental sulfur was activated by ODE, the formation reaction of CdS followed closely. 2-tetradecylthiophene from the activation of S by ODE and fatty acids from the formation reaction of CdS were found to be the only soluble side products. The overall reaction stoichiometry further suggested that oxidation of each ODE molecule generated two molecules of H(2)S, which in turn reacted with two molecules of cadmium carboxylate molecules to yield two CdS molecular units and four molecules of fatty acids. In comparison to alkanes, octadecene was found to be substantially more active as a reductant for elemental sulfur. To the best of our knowledge, this is the first example of quantitative correlation between chemical reactions and formation of high-quality nanocrystals under synthetic conditions. To demonstrate the importance of such discovery, we designed two independent and simplified synthetic approaches for synthesis of CdS nanocrystals. One approach with its reaction temperature at the critical temperature of S activation (180 degrees C) used the same reactant composition as the classic approach but without any hot injection. The other approach performed at an ordinary laboratory temperature (<= 100 degrees C) and in a common organic solvent (toluene) was achieved by addition of fatty amine as activation reagent of elemental sulfur.