Propagation rate constants (kp) for the polymerization of methyl methacrylate (MMA) and styrene (STY) were determined by using a recently developed experimental technique. The procedure utilized a pulsed laser UV light source for the polymerization, and gel permeation chromatography (GPC) was used to analyze the products. The average kpvalues obtained were 294 ± 9 and 78 ± 6 L mol-1s-1 for MMA and STY, respectively at 25 °C. These kp values were found to be independent of chain length (v), initiator type, and concentration. The inclusion of methanol, ethylbenzene (to STY), ethyl acetate (to MMA), or poly-STY at 0.5% w/v as additive to the polymerization mixture had no significant effect on kp.