Abstract: The photopolymerization of acrylonitrile and methyl methacrylate initiated by O-acyl-α-oximinoketone was studied under irradiation at 365nm and 313nm. The kinetic behavior was quite different in high absorbance and in low absorbance systems. In low absorbance, the initiator exponent measured was 0.5, while in high absorbance, the rate of polymerization and molecular weights of the polymers were all independent of the initiator concentrations. The greater the extinction coefficient of the initiator, the lower the concentration at which initiator exponent is zero power.