The photodegradation of thin films of poly(p-tert-butylstyrene) with 254-nm radiation under high vacuum has been studied. The principal gaseous product is hydrogen, but smaller amounts (in decreasing order of yield) of methane, ethane, and isobutane are also formed, indicating that fission of bonds in the tert-butyl group is also involved. Ultraviolet and visible spectra of degraded films indicate the presence of unsaturated groups, initial rates of formation of these being considerably higher than those for polystyrene films. Solubility data indicate that crosslinking apparently occurs less readily than in the case of polystyrene, and this is interpreted in terms of the enhanced stability (due to the +I effect of the tert-butyl group) of the radicals formed by chain scission and also of the inhibiting steric effect of the tert-butyl group, particularly on crosslinking reactions involving the benzene rings. Quantum yields for the gaseous products and for chain scission and crosslinking were determined, and the mechanism of the various reactions are discussed.