Thermosensitive semiinterpenetrating polymer networks (semi-IPNs) composed of cross-linked poly(N-isopropylacrylamide) (NiPAAm) and linear poly(ether(urethane-urea) (Biomer) were obtained via UV-initiated solution polymerization. The semi-IPNs exhibited negative thermosensitivity, i.e., lower swelling levels with increasing temperature. The incorporation of a relatively small content of Biomer (up to 10 wt %) strongly influenced the mechanical properties, equilibrium swelling, and deswelling kinetics of synthesized networks. The semi-IPNs exhibited greater mechanical strength compared to the cross-linked poly(NiPAAm). Equilibrium swelling levels of the semi-IPNs at low temperatures were markedly decreased due to hydrophobic contribution of Biomer and higher apparent effective cross-linking densities of these networks. The gel collapse point, related to the lower critical solution temperature of poly(NiPAAm), was not affected. The semi-IPNs showed much faster deswelling rates compared to the cross-linked poly(NiPAAm). It was hypothesized that the presence of Biomer prevented the formation of a skin-type layer which normally retards the deswelling process of cross-linked poly(NiPAAm). Loading and release of heparin, a model macromolecule, was studied as a function of temperature and Biomer content in semi-IPNs. The partition coefficients of heparin within the networks decreased with increasing temperature and Biomer content. Similarly, a linear relationship between partition coefficients and equilibrium swelling in loading solutions was found for all synthesized networks. Heparin release profiles correlated with deswelling kinetics of cross-linked poly(NiPAAm) and NiPAAm/Biomer semi-IPNs. Release profiles were in agreement with the proposed mechanism of solute release from swollen thermosensitive gels.