Polystyrenes vere prepared by keeping temperature and the amount of phosphorus pen- toxide constant, but changing the monomer concentration. Rate of polymerization was found to be proportional to the second power of the monomer concentration. The number average degrees of polymerization were determined by the vis- cosity measurements. From the slope of the graph plotted 1/DP versus 1/[M]O, the ratio of (ktr_g [S]o + kt)/kp was found to be 0.11*. Dependence of the rate of polymerization of styrene on the concentration of phoshorus pentoxide was observed as the first order reaction in the experiments conducted at three different temperatures. At these temperatures the overall rate constants were determined as follows: at + 20 °C; 10.0 x 10-4 l2 mor2 sec-1 at 4~ 5 °C; 7.6 x 10-4 l2 mol-2 sec-1 at — 20 °C; 0.9 x 10-4 l2 mol-2 sec-1 and the total activation energy (Ea) was found to be 46.1 kj mol-1. Polymerization reaction were performed by adding of varying amounts of water into the reaction vessel. The rate of the polymerization increased as the amount of water increased; pas- sing throungh a nıaximum, and then started to fail as the amount of water increased. At the maximum rate the ratio of [H 20 J / [P 20 5 ] was computed to be 0.69. Although the water effected the rate of polymerization as indicated above, the molecular weights