The chain transfer behavior of fractionated commercial tertiary mercaptans was investigated in batch and semicontinuous emulsion polymerization of styrene over the entire monomer conversion range. Four mercaptans were obtained by fractionation, which contained 9, 11, 12, and 13 carbons, respectively. The effect of the mercaptans was evaluated in terms of the consumption rates of the monomer and the chain transfer agents, the number average degree of polymerization, DP(n), and the polydispersity index, I, of the polymer. The batch experiments showed that the chain transfer efficiency decreases with increasing carbon number, which is due primarily to a lower diffusion rate of the chain transfer agent to the reaction sites (growing latex particles) through the aqueous phase. The partitioning ratio of the chain transfer agents between the aqueous phase and the monomer droplets also contributes, to a lesser extent, to the efficiency. The number average degree of polymerization and the polydispersity index are primarily controlled by the ratio of the mercaptan consumption rate over that of the monomer. In order to obtain a polymer with a constant DP(n) and a narrow I, this ratio should be as close to unity as possible, as is the case for C-11. Otherwise, too high a ratio causes a severe increase in DP(n) at the end of polymerization, and too low a ratio leads in the opposite direction. The semicontinuous experiments confirmed the batch results.