The photodegradation of polymethyl methacrylate films in vacuum and in air by UV radiation from a medium-pressure mercury lamp has been investigated at room temperature. Changes in weight in terms of intrinsic viscosities, the formation of volatile substances, and the increase in the UV absorption of the films were followed as a function of energy absorbed. Quantum yields for random chain scission in polymethyl methacrylate are about 0.04 and 0.02 in vacuum and in air, respectively. Quantum yields in vacuum are the same at 2 and 0.02 microns pressure. The quantum yield for methyl formate is approximately that for chain scission the formate is accompanied by larger amounts of methanol and very small quantities of monomer. Increased UV absorption occurs after extensive degradation in either air or vacuum and is due to chromophores formed in the polymer rather than in low molecular weight fragments.