Hydrogels based on semi-interpenetrating network (semi-IPN) combining alginate-Ca2+ (matrix) with poly(N-isopropyl acrylamide) (PNIPAAm) were prepared and characterized in order to determine their affinity to water and their permeability to orange II as a function of temperature. Membranes of these hydrogels were obtained by gelation of the aqueous solution of alginate and PNIPAAm by the addition of CaCl2. The presence of PNIPAAm chains inside the hydrogels alters the water affinity when compared to the pure alginate-Ca2+ hydrogels. Although the water uptake capability decreases above 32 degreesC (Low Critical Solution Temperature (LCST) of PNIPAAm in water), no shrinking of the semi-IPN hydrogels during the phase separation of the PNIPAAm was observed. The permeability of orange II as a function of temperature decreases at 32 degreesC and shows a dependence on the molar mass of the alginate. The partition coefficient of orange II in the hydrogel membrane, relative to water, decreases by increasing the temperature and its permeability follows a similar behavior. It was proposed that above the LCST of PNIPAAm the Alginate-Ca2+ networks mechanically support the collapsed PNIPAAm chains and the diffusion of orange II is minimized. The collapsing process may be followed by the formation of a complex between the carboxylic side groups of alginate and -NH-R groups of PNIPAAm. It would expose the isopropyl groups of PNIPAAm chains, providing a hydrophobic environment that minimizes the interaction between the dye and the polymeric matrix.