The effects of end-group balance and moisture level on melt-phase polycondensation reactions were investigated using nylon 612. The polycondensation reaction was determined to be first-order in amine ends and first-order in carboxyl ends at the relatively high temperature (284 degrees C) and low water concentration conditions (0-0.002 mass fraction) studied, which are encountered in the later stages of nylon polymerization processes. Using data from this study and the nonisothermal data of Schaffer et al. (Chemical Pathways and Kinetics of the Later Stages of Nylon Polymerization Processes. Ph.D. Thesis, Queen's University, Kingston, Ontario, Canada, 2003; Experimental Study and Modeling of Nylon Polycondensation in the Melt Phase. Ind. Eng. Chem. Res. 2003, 42, 2946), a mathematical model was developed that can accurately describe changes in both the polyamidation reaction rate and the apparent equilibrium constant, with changing water concentration and temperature.