The photodegradation of films of poly chain-chlorostyrene (PCCS) and poly chain-bromostyrene (PCBS) achieved by exposure to 254-nm radiation under dynamic vacuum was studied. The minor gaseous products obtained were hydrogen halides and monomer. The polymers were investigated by ir, gel permeation chromatography, and microanalysis. Both polymers undergo main chain scission. The ir spectra of the degraded polymers indicate the presence of unsaturated species. An initial alpha-carbon-halogen bond fission is required to account for the present reaction products. Possible mechanism of the various photoreactions are discussed.