Hydrogel materials are used in many technologies, including biomaterials for contact lenses and wound dressings, and membranes for pervaporation processes and fuel cells. The evaporation-induced diffusion of water through the hydrogel material is a significant factor in determining the performance of hydrogel materials in these applications. For this reason, it is important to characterize the mass transfer properties of hydrogels with respect to the transport of water. We report a simple, accurate experimental technique that makes this characterization possible. The technique measures the chemical potential drop across a hydrogel membrane with one side exposed to a vapor phase and the other to a liquid water reservoir or to a second vapor phase with higher water vapor content than the first. The chemical potential drop is determined by using a heat and mass balance of the multi-phase system and a psychrometric calibration of mass transport in the vapor phase above the membrane. We determine the response of the hydrogel membrane to changing chemical potential drops by measuring the steady-state flux of water vapor through the membrane. This response is summarized by global mass transfer functions. We also calculate the diffusivity of water within the membrane using the measured steady-state flux and chemical potential drop together with Flory-Rehner polymer-solvent mixture thermodynamics and various transport models. Experiments are performed with poly(hydroxyethyl methcacrylate) membranes. We test membranes of varying thickness contain varying amounts of cross-linking agent to determine the effects of these attributes on transport properties. Experimental results reveal that the model-dependent diffusivity of water in the material decreases as the driving force for diffusion is increased. The decreasing diffusivity is explained by a water desaturation-induced glass transition of the polymer network constituting the solid fraction of the hydrogel, the occurrence of which is confirmed using Differential Scanning Calorimetry at controlled relative humidity.