Transparent propylene glycol monoacrylate (PGMA) hydrogels become reversibly opaque when the temperature is rapidly increased to above 30°C. Water permeation, driven by a hydrostatic pressure difference through three PGMA membranes differing in water content and thickness was determined at several temperatures. At room temperature, the permeability to water is relatively low (less than 10 × 10−16 cm.2) and is dominated by a diffusion mechanism. When the temperature is suddenly increased, the water permeation rises quickly (up to fifteen times or more depending on the temperature), and viscous flow predominates over diffusive flow. The rapid flow slows down after some time at the same temperature, or on lowering the temperature, and may be interpreted as caused mainly by the reversible transformation of the membrane from the heterogeneous to the homogeneous form. When the temperature is increased gradually, the permeability of the membrane is in the same order of magnitude as that found at room temperature, because no phase separation occurs. The permeation of water decreased in successive measurements made under the same pressure gradient. An increase of the pressure difference also resulted in decreased permeation, indicating that water was pressed out of the gel. Dehydration of the gel caused by increases in temperature (exothermic swelling) and in pressure and changes in the viscosity of water with temperature are some of the factors which are considered in the interpretation of the results.