Semi-interpenetrating polymer networks (semi-IPNs) based on crosslinked polyurethane (PU) and linear polyvinylpyrrolidone (PVP) were synthezised, and their thermal and dynamic mechanical properties and dielectric relaxation behavior were studied to provide insight into their structure, especially according to their composition. The differential scanning calorimetry results showed the glass transitions of the pure components: one glass-transition temperature (T-g) for PU and two transitions for PVP. Such glass transitions were also present in the semi-IPNs, whatever their composition. The viscoelastic properties of the semi-IPNs reflected their thermal behavior; it was shown that the serni-IPNs presented three distinct dynamic mechanical relaxations related to these three T-g values. Although the temperature position of the PU maximum tan delta of the alpha-relaxation was invariable, on the contrary the situation for the two maxima observed for PVP was more complex. Only the maximum of the highest temperature relaxation was shifted to lower temperatures with decreasing PVP content in the semi-IPNs. In this study, we investigated the molecular mobility of the IPNs by means of dielectric relaxation spectroscopy; six relaxation processes were observed and indexed according the increase in the temperature range: the secondary beta-relaxations related to PU and PVP chains, an alpha-relaxation due to the glass-rubber transition of the PU component, two alpha-relaxations associated to the glass-rubber transitions of the PVP material, and an ionic conductivity relaxation due to the space charge polarization of PU. The temperature position of the a-relaxation of PU was invariable in semi-IPNs, as observed dynamic mechanical analysis measurements. However, the upper a-relaxation process of PVP shifted to higher temperatures with increasing PVP content in the semi-IPNs. We concluded that the investigated semi-IPNs were two-phase systems with incomplete phase separation and that the content of PVP in the IPNs governed the structure and corresponding properties of such systems through physical interactions. (C) 2003 Wiley Periodicals, Inc.