Water-vapor absorption of polyurethane/Al bonds was studied, and the weight gain of samples after water-vapor exposure was measured to examine the effect of the polyurethane crystalline interphase formed at the Al surface on water-vapor absorption. Water-vapor absorption was found to be independent of the crosslink density of the polyurethanes. Instead, it was found that the microstructural characteristics of the spherulitic interphase, a mixture of crystallites and amorphous material formed on the Al surface, strongly affects water-vapor absorption: As the area fraction and the size of spherulites for the films increases, the amount of absorbed water vapor initially increases rapidly and then remained constant. The water vapor absorbed and condensed at spherulite boundaries consisting of voids and defects was created due to volume contraction during polymer crystallization. (C) 2003 Wiley Periodicals, Inc.