The photoinitiated (at wavelength I = 365 nm) terpolymerization of acrylonitrile, maleic anhydride and styrene in the presence and/or absence of additives such as ZnC1, or N,N-dimethylaniline (DMA) in acetone was studied, and the relative reactivities R of maleic anhydride and acrylonitrile towards a styryl radical were determined. In the presence of ZnClz and/or DMA, the relative reactivities R are by about 40% lower than the value of R (= 13.3 k 2,l) for the terpolymerization system without additives.