Semi-interpenetrating polymer networks (semi-IPNs) were prepared from linear polyurethane (PUR) and polycyanurate (PCN) networks. Wide-angle X-ray scattering measurements showed that the IPNs were amorphous, and differential scanning calorimetry and small-angle X-ray scattering measurements suggested that they were macroscopically homogeneous. Here we report the results of detailed studies of the molecular mobility in IPNs with PUR contents greater than or equal to 50% via broadband dielectric relaxation spectroscopy (10 Ϫ2 -10 9 Hz, 210 -420 K) and thermally stimulated depolarization current techniques (77-320 K). Both techniques gave a single ␣ relaxation in the IPNs, shifting to higher temperatures in isochronal plots with increasing PCN content, and provided measures for the glass-transition temperature (T g ) close to and following the calorimetric T g . The dielectric response in the IPNs was dominated by PUR. The segmental ␣ relaxation, associated with the glass transition and, to a lesser extent, the local secondary  and ␥ relaxations were analyzed in detail with respect to the timescale, the shape of the response, and the relaxation strength. The ␣ relaxation became broader with increasing PCN content, the broadening being attributed to concentration fluctuations. Fragility decreased in the IPNs in comparison with PUR, the kinetic free volume at T g increased, and the relaxation strength of the ␣ relaxation, normalized to the same PUR content, increased. The results are discussed in terms of the formation of chemical bonds between the components, as confirmed by IR, and the reduced packing density of PUR chains in the IPNs.