The melt-phase kinetics and equilibrium of nylon polycondensation and the mass transfer of water under conditions of high temperature and low water concentration were investigated by experimental study and mathematical modeling. Nylon 612 was used as a more thermally stable alternative to nylon 66 in a novel reactor system. The water concentration in the melt was varied by altering the composition of a steam/nitrogen mixture bubbled through the stirred molten polymer. Experimental data for the time evolution of carboxylic acid and amine end-group concentrations, water concentration, and inherent viscosity of the polymer samples were collected and compared with the predictions of two mathematical models from the literature. A simplified mathematical model describes the experimental data as well as or better than the literature models and accounts for the solubility of water in the nylon melt, the mass transfer of water, the rate of reaction, and the variation of the apparent equilibrium constant with the water concentration.