Effects of sodium hypophosphite (SHP) catalyst and water on nylon 612 melt-phase polycondensation reactions were investigated using dynamic experiments with different catalyst levels (33, 113 and 249 ppm). The water concentration in the nylon melt was varied by altering the composition of a steam/nitrogen mixture bubbled through the molten polymer. Carboxyl and amine end-group concentrations were determined for polymer samples collected during the experiments. A model was developed to describe the kinetics of nylon polycondensation using the SHP catalyst in the high-temperature and low water operating region that corresponds to the final stages of industrial melt-phase polyamidation processes.