The chain transfer to monomer constants (Cm) in styrene bulk polymerizations have been determined in the temperature range between 25 and 90 °C using both thermal polymerization and the novel pulsed laser polymerization (PLP) with extended dark times. This novel technique determines transfer to monomer rate coefficients by combining a pulsed laser setup with a rotating reactor/cuvette assembly. The resulting transfer controlled molecular weight distributions (MWD) were analyzed by the well-known Mayo and chain length distribution (CLD) methods. The results of both experimental methods agree well, which indicates that PLP with extended dark times is a reliable technique to determine transfer to monomer constants. For this reason, we report average values for the activation parameters determined from both thermal and extended dark times PLP experiments: Ea = 21.6 kJ mol-1 and A = 0.22 L mol-1 s-1. These numbers are in excellent agreement with those reported in the literature.