The photodegradation of thin films of poly-<alpha>-methylstyrene by near ultraviolet light has been studied in vacuum at 27<deg> and 115<deg> C. Changes taking place during photolysis were followed by determining monomer formation by ultraviolet spectroscopy, residue molecular weight through a measurement of intrinsic viscosity, and volatile products by mass spectroscopy. Quantum yields for chain scission and monomer formation were calculated. The main features of the photolysis were found to be random chain scission in the polymer with subsequent depolymerization of the radicals formed to yeild only small amounts of monomer; the kinetic chain length is on the order of 15 in the temperature range studied.