Hydrophilic three-dimensional methacrylate polymer networks(hydrogels) were prepared from 2-hydroxyethyl methacrylate and 2,3-dihydroxypropyl methacrylate. Highly isotactic poly (2-hydroxyethyl methacrylate) and highly syndiotactic poly(2-hydro-xyethyl methacrylate) were also studied. The sorption of water vapor in the different tactic polymers was measured as a function of water activity. The results were interpreted in terms of Anderson''s modified B.E.T. theory and the Hailwood-Horrobin theory of sorption. The former fitting to the experimental isotherms is up to 0.7 equilibrium relative humidity, while the solution theory of Hailwood-Horrobin gives a somewhat better fit to most of the experimental isotherms over the entire range of equilibrium relative humidity. Water vapor sorption by the methacrylate polymers is affected by tacticity and the number of hydrophilic sites in molecules. The amount of water vapor sorbed by isotactic poly(2-hydroxyethyl methacrylate) was found to be greater than that by syndiotactic poly(2-hydroxyethy1 methacrylate). Theoretical water sorption models and analysis of the data suggest that isotactic poly(2-hydroxyethyl methacrylate) exists in a surface configuration and structure which provides a larger number of water binding sites in the syndio- or hetero-tactic polymers. Poly(dihydroxypropyl methacrylate) water sorption data and analysis is consistent with its higher hydrophilicity due to the presence of 2-hydroxy1 group per repeat unit. These data are also consistent with thermal analysis studies of bound water in fully hydrated systems.