Semiinterpenetrating polymer networks (semi-IPNs) based on polyurethane (PU) and polyvinylpyrrolidone (PVP) have been synthesized, and their thermodynamic characteristics, thermal properties, and dynamical mechanical properties have been studied to have an insight in their structure as a function of their composition. First, the free energies of mixing of the two polymers in semi-IPNs based on crosslinked PU and PVP have been determined by the vapor sorption method. It was established that these constituent polymers are not miscible in the semi-IPNs. The differential scanning calorimetry results evidence the T-g of polyurethane and two T-g for PVP. The dynamic mechanical behavior of the semi-IPNs has been investigated and is in accordance with their thermal behavior. It was shown that the semi-IPNs present three distinct relaxations. If the temperature position of PU maximum tan delta is invariable, on the contrary, the situation for the two maxima observed for PVP is more complex. Only the maximum of the highest temperature relaxation is shifted to lower temperature with changing of the semi-IPNs composition. It was concluded that investigated semi-IPNs are two-phase systems with incomplete phase separation. The phase composition was calculated using viscoelastic properties.