Deviations from the “ideal” behavior have been observed in the polymerization of styrene in acetone. Both monomer and initiator exponents differ from the expected values. The poor thermodynamic properties of acetone as a solvent for the polystyrene chain seem responsible for the decrease of the initiation rate exponent, owing to the presence of a noticeable primary termination reaction. The strong effect of the reaction medium on monomer exponent can be related to the electron-donating tendency of acetone. The sharp decrease of the fractional rate of polymerization as a function of dilution has been interpreted in terms of lowering of the propagation rate constant, presumably due to solvent complexation of the growing chains.