The photodegradation by 2537 A radiation of four methyl methacrylate/acrylonitrile copolymers with molar ratios 410/1, 40[1, 16/1 and 8/1 has been studied at 160 Β°. Volatilization of the copolymers, to give methyl methacrylate almost exclusively, is accompanied by a rapid decrease in the molecular weight of the residue. The relationship between the molecular weight of the residue and the extent of volatilization suggests that chain scission is associated with acrylonitrile units; this is confirmed by the observation that the rate of chain scission increases with acrylonitrile content. From molecular weight and volatilization data it has been calculated that 1710, 728, 512 and 256 methyl methacrylate molecules respectively are produced per chain scission in the 410/1, 40/1, 16[1 and 8/1 copolymers. These findings are interpreted as follows. After scission near acrylonitrile units, the resulting radicals may depolymerize as far as the next acrylonitrile unit in the chain. Because of the high viscosity of the medium and depending upon the distance apart of the acrylonitrile units, a proportion of the radicals recombine thereby accounting for the comparatively large amount of monomer produced per chain scission in the copolymers richer in acrylonitrile. Those pairs of radicals which do not recombine probably take part in transfer reactions. The principal differences between the photoreaction and the thermal reaction, which was discussed in the previous paper, are emphasized.