Thermal and mechanical properties were measured for solvent-cast films of cellulose acetate propionate plasticized with triphenyl phosphate. Probabilistic models for the glass transition temperature, tensile heat deflection temperature, tensile modulus, break stress, and initial tear strength as functions of weight-average molecular weight and level of plasticizer were developed. The glass transition temperature and the tensile heat deflection temperature were modeled well by functions of the level of plasticizer as the only variable, whereas models for the tensile modulus, break stress, and inital tear strength included molecular weight. Only the break stress showed a statistically significant term of interaction between molecular weight and level of plasticizer. This term was such that, for unplasticized films, increasing the molecular weight increased the break stress, whereas, for films formulated with 20 pph plasticizer, increasing the molecular weight decreased the break stress. Physical explanations for this interaction term are discussed.