In this study, two typical modeling samples composed of vinyl acetate (VAc) and vinyl alcohol(VA) copolymer were prepared in order to investigate the relationship between polymer structure and water uptake. One consisted of homogeneously acetylated samples in which the hydroxyl group and acetoxyl group were distributed randomly, and the other consisted of the heterogeneously acetylated samples which were composed of block copolymer of poly(VA) and poly(VAc-VA). The sorption isotherms of water vapor by those samples with various contents of hydroxyl and acetoxyl groups were measured. The BET adsorption theory was modified for the system which has two different kinds of energy levels in water adsorption sites, that is, a hydroxyl group and an acetoxyl group, and applied in order to analyze the obtained sorption isotherms, The homogeneously acetylated VAc/ VA copolymer was amorphous polymer, and the heterogeneously acetylated VAc/VA copolymer was a partially crystallized polymer. The number of adsorption sites in the homogeneously acetylated samples was larger than that in the heterogeneously acetylated sample. On the other hand, the interaction energy between the hydroxyl group and water in the heterogeneously acetylated sample was much larger than that in the homogeneously acetylated sample based on examination with the modified BET equation. These results imply that the poly(vinyl alcohol) crystal has a high adsorption energy,