The rates of initiation Ri of polymer chains in styrene and methyl methacrylate have been measured for four different catalysts. The results for styrene at 60°C are: Ri = 1.28 × 10−5[azo]; Ri = 2.96 × 10−6[Bz2O2]; Ri = 6.12 × 10−7[CHP]; Ri = 9.96 × 10−5[BHP]. Azo, Bz2O2, CHP, and BHP refer to 2-azobisisobutyronitrile, benzoyl peroxide, cumene hydroperoxide, and tert-butyl hydroperoxide, respectively. Rates are expressed in moles per liter per second. The corresponding results for methyl methacrylate at 60° are: Ri = 1.42 × 10−5[azo]; Ri = 6.62 × 10−6[Bz2O2]; Ri = 3.92 × 10−7[CHP]; Ri = 7.30 × 10−8[BHP]. Catalyst efficiencies for azo and Bz2O2 are shown to be nearly unity in both monomers. Chain transfer constants were calculated for all catalysts in both monomers. Photopolymerization was shown to proceed via chains growing in one direction only. The polymerizations were all carried out in the pure monomers (bulk polymerization) with exclusion of oxygen and other inhibitors.