A study has been made of the polymerization of styrene in the liquid phase, initiated by acetyl radicals from the photolysis of diacetyl. The rate of consumption of monomer was proportional to the square root of the rate of light absorption; the rate of consumption of diacetyl was proportional to the first power of the rate of light absorption. The ratio of the rate constant for chain propagation to the square root of the rate constant for chain termination was found to be 3.9 × 10−3 liter1/2 mole−1/2 sec.−1/2 at 20°C. The quantum yield of diacetyl consumption was 4.0 × 10−3. By relating the kinetic data to the observed intrinsic viscosities, the exponent of the number average molecular weight in the modified Staudinger equation was found to be 0.65. The values of the corresponding K factors for homogeneous and heterogeneous polymers were 6.5 × 10−3 and 8.2 × 10−3, respectively, when the assumption was made that chain termination was by recombination; for chain termination by disproportionation the values were 8.7 × 10−3and 12.9 × 10−3, respectively, for concentrations in base moles per liter.