The interdiffusion of polymer chains during film formation was correlated with the development of tensile strength of the resulting films. After an apparent induction period, tensile strength increases with annealing time. Full tensile strength was achieved at a penetration depth of 30-35 A, comparable to the radius of gyration of the critical entanglement molecular weight of polystyrene. Two critical parameters in the interdiffusion process of film formation from latex particles are proposed: the location of chain ends and the dimensionless ratio of the polymer chain's radii of gyration to the latex particle's radius