The polymerization rates of o- and p-chlorostyrenes have been determined under a variety of reaction conditions. These monomers polymerize much more rapidly than styrene thermally with peroxide, or high energy radiation. With no initiator the polymerization of chlorostyrene follows a behavior similar to styrene, i.e., the reaction exhibits a maximum rate during the initial stage of the reaction. With initiator, the form of the rate curve changes and acceleration is observed in the midconversion range. The ratio of the accelerated rate to the initial rate increases with initiator concentration. Mixtures of chlorostyrene with small amounts of certain difunctional monomers (e.g., divinylbenzene, ethylene dimethacrylate, and biacrylophenone) copolymerize much more rapidly than the chlorostyrene polymerizes alone. Polyester resins containing maleic or fumaric unsaturation copolymerize much more rapidly with chlorostyrene than with styrene under identical reaction conditions.