The hydrolytic stability of both isotropic and oriented semicrystalline poly(ethylene naphthalene-a,g-dicarboxylate), PEN, films in saturated steam at temperatures 120 - 160 degrees C is reported. The kinetics of hydrolytic degradation of the semicrystalline PEN samples, together with semicrystalline PET, was studied by determining the rates of formation of carboxyl ends using infrared spectroscopy. Water vapor diffusion rates and water vapor solubilities of PEN and PET samples at various temperatures were also measured. The water vapor diffusion rates of PEN were found to be 2-3 times slower than those of PET. However, the water vapor diffusion rates of both PEN and PET were 3-4 orders of magnitude greater than their hydrolytic degradation rates. The hydrolysis in all the cases is therefore believed to be a chemical reaction rate controlled process. PEN, especially when oriented, showed better hydrolytic stability than PET. A half-order autocatalytic reaction mechanism with the carboxyl end groups acting as the catalyst gave the best explanation of the experimentally measured degradation rates, and from this analysis hydrolytic degradation rate constants were derived for PEN and PET.