AbstractFive twelve‐carbon mercaptans of definite structure are prepared and used to demonstrate that the rate of diffusion of mercaptan regulators in emulsion polymerization varies inversely with the length of the longest straight chain of the mercaptan structure (including the sulfur). In a given class of mercaptans (primary, secondary, or tertiary) the mercaptan consumption during polymerization varies directly with the diffusion rate. Solution viscosities of the resulting polymers are in accord with these finding