Tertiary octyl, decyl, dodecyl, and hexadecyl mercaptans were investigated as modifiers in GR-S polymerizations.The rate of disappearance of these modifiers followed a first-order rate equation during the first half of the polymerization, in accordance with the theory of chain-transfer reactions. During the latter part of the reaction, an increased rate of disappearance generally occurred. A mechanism is suggested.The rate of disappearance increased with decreasing chain-length of the mercaptan. The intrinsic viscosities and molecular weight distributions of the polymers obtained at different conversions were related to the activity of each mercaptan as a modifier. Values for the regulating index of each mercaptan were calculated.The slow rate of disappearance of tertiary hexadecyl mercaptan leads to the formation of polymer that is relatively homogeneous in molecular weight, and this mercaptan is therefore a desirable modifier. However, after 50% polymerization, the regulator action shows a marked acceleration, leading to the formation of progressively lower molecular weight polymer and resulting in a spread in the molecular weight distribution.