Abstract: The photopolymerization of acrylonitrile and methyl methacrylate initiated by O-acyl-α-oximinoketones was studied under the irradiation at 313 and 365nm. The kinetic behavior was quite different in high absorbance and low absorbance systems. In higher absorbance the rate of polymerization and the molecular weights of the polymers were all independent of the initiator concentrations. The greater the extinction coefficient of the initiator,the lower the concentration with initiator exponent being zero power.