Water-vapor transport in poly( acrylic acidi (PAA), poly(N-vinyl-2-pyrrolidone) (PVPo), and their 2 : 1 interpolymer complex was measured between 30 and 50 degrees C. It was found that the formation of interpolymer hydrogen bonding not only caused the T-g of the complex to be higher than those of the individual components, but also affected vapor-sorption behaviors. In the high vapor pressure region (P-r > 0.5), sorption isotherms in pure PAA and PVPo follow the Flory-Huggins solution theory and the sorbed water molecules may form clusters around the COOH groups in PAA and C=O groups in PVPo. However, sorption in the complex is almost linear for the entire range of water activities. The binary polymer-polymer interaction parameter between PAA and PVPo in the complex, chi(PAA-PVPo), is less than zero, which is indicative of favorable interaction between PAA and PVPo. At lower water activities, sorption behaviors for water in the three polymers can be approximated by Henry’s law and vapor-diffusion coefficients can be ranked as PVPo > complex > PAA. The water vapor diffusion coefficients in each system decrease with increasing P-r in the region of P-r > 0.5 because the proportion of immobilized water molecules increases with vapor concentration.