Photodegradation of poly (p-chlorostyrene) (PPCS) acid poly(p-bromostyrene) (PPBS) films was studied using 254nm UV under dynamic vacuum at room temperature. The polymers were examined by IR, and NMR spectroscopy, gel permeation chromatography, and microanalysis. Solubility data confirms that both polymers undergo chain scission and crosslinking simultaneously. Evidence of scission of the C-Ph bonds in the chain backbone has been found. Possible mechanisms of the various reactions are discussed.