Semi-interpenetrating polymer networks (semi-IPN) of linear polyurethane (LPU) and heterocyclic polymer network (HPN) were characterized by IR spectroscopy, small-angle X-ray scattering, differential calorimetry with diathermal shells (temperature interval 130-500 K), and dynamic mechanical analysis (temperature interval 150-500 K and frequency range 0.3-30 Hz). The main results obtained may be summarized as follows: 1) The affinity of HPN towards the soft LPU microphase is responsible for the absence of large-scale heterogeneity of semi-IPNs on their base (at least, at low HPN/LPU ratios). 2) Phenomenological analysis of small-scale, Arrhenius-like subglass relaxations (gamma(1) and gamma(2) in LPU and in HPN, respectively) revealed little interference of HPN with relaxation centers (RCs) of LPU in semi-IPNs, while the local environment of RCs in HPN, on the contrary, strongly depended on semi-IPN composition.