An unsteady reaction–diffusion model has been developed to represent the polymerization of a nylon 6,6 prepolymer in a stationary film. A realistic kinetic and equilibrium description is incorporated by the use of apparent rate and equilibrium constants that are composition dependent. The model is suitable for evaluating the effect(s) of changes in throughput (film thickness), temperature, and catalyst activity on the performance of an idealized film reactor. The model is characterized by a single dimensionless parameter, the Thiele modulus. The composition-dependent kinetic constants are important for accurate prediction of molecular weight evolution in the film. Below a threshold value of the Thiele modulus, molecular weight generation is reaction-controlled, while above another threshold value it is diffusion-controlled.