A stochastic model has been proposed and partially validated for predicting the service life of a nominal population of polymethyl methacrylate (PMMA) films subjected to photolytic degradation. Service life is defined as the time after which an acceptable portion of a nominal population of PMMA films subjected to photodegradation has failed; failure occurs when a performance property of the film falls below a predesignated minimum value. It was demonstrated that the expected change in two performance properties of the films was functionally related to the expected number of chain scissions. -- AATA