Abstract 1. Solution polymerizations of styrene, p-methyl-, p-ethyl-, p-tert-butyl-, m-methyl-, 2,4-dimethyl-, 2,5-dimethyl- and 3,4-dimethylstyrene were carried out at 50 and 70°C by ABIN in benzene, and the rates of polymerization and [η]’s of the polymers were determined. For 3,4-dimethylstyrene, the same equation as for styrene was obtained: R 0=k[M] [ABIN]1⁄2 2. Rate constants of initiation of the polymerizations of styrene and p-methylstyrene were determined, according to the FeCl3-method as follows: k i=0.67×10−5 (sec−1) (for p-methylstyrene) k i=0.76×10−5 (sec−1) (for styrene) 3. Polymerizations of styrene and p-methylstyrene were carried out at 70°C by the Bz2O2-DMA system, and the same equation as styrene was derived for p-methylstyrene as follows: R 0=k′[Bz2O2]1⁄2[DMA]1⁄2 4. Polymerizations of styrene, p-methyl-, and dimethylstyrenes were carried out at 50 and 70°C by ABIN-DMA system, and the results for the rate of polymerization and the value of [η] were discussed. A correction was made for the results previously obtained. 5. The effect of DMA on the induction period for the polymerization of p-methylstyrene was studied at 60°C. In the case of polymerization with FeCl3 in DMF, no effect of DMA on the induction period was observed, while in the case of that with p-benzoquinone in benzene, the induction period was decreased with the amount of DMA.