Three series of novel semi-interpenetrating polymer networks, based on crosslinked poly(N-isopropylacrylamide), PNIPA, and different amounts of the linear poly(N-vinylpyrrolidone), PVP, were synthesized to improve the mechanical properties and thermal response of PNIPA gels. The effect of the incorporation of the linear PVP into the temperature responsive networks on the temperature-induced transition, swelling/deswelling behavior, and mechanical properties Was Studied. Polymer networks with four different crosslinking densities were prepared with varying molar ratios (25/1 to 100/1) of the monomer (N-isopropylacrylamide) to the crosslinker (N,N'-methylenebisacrylamide). The hydrogels were characterized by determination of the equilibrium degree of swelling, the dynamic shear modulus and the effective crosslinking density, as well as tensile strength and elongation at break. Furthermore, the deswelling kinetics of the hydrogels was studied by measuring their water retention capacity. The inclusion of the linear hydrophilic PVP in the PNIPA networks increased the equilibrium degree of swelling. The tensile strength of the semi-interpenetrating networks (SIPNs) reinforced with linear PVP was higher than that of the PNIPA networks. The elongation at break of these SIPNs varied between 22%, and 55%, which are 22 - 41% larger than those for pure PNIPA networks. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 1593-1603, 2009