We have developed a new model for interfacial polymerization (IF) which predicts both the kinetics of film formation as well as the molecular weight distribution of the polymer. The model predictions are validated with experimental data for unstirred IP in the Nylon 6-10 system. The model brings out the feature of the precipitation of high-molecular-weight polymer in the beginning and the subsequent precipitation of lower oligomers resulting in a reduction in molecular weight and increase in polydispersity. The model also predicts the experimentally observed maximum in the molecular weight/intrinsic viscosity of the polymer film with variation in the acid chloride concentration. The model provides an important first step in design and prediction of properties of membranes produced by interfacial polymerization.