The process of formation of polymer blends based on linear polyurethanes (PU) and hardening polyepoxide (PEp) both without and with addition of epoxy-urethane copolymer (EpUC) was studied. Correlation between the phase separation initiation time and the degree of conversion of epoxy groups was ascertained. It was shown that the process of formation of PU/PEp blends involves an extremal character of variation of the degree of conversion of epoxy groups and of the phase separation initiation time with the composition of the system. The addition of EpUC to the hardening system caused a substantial change of the phase separation initiation time and changed the physical and mechanical characteristics of the resulting material. The influence of the molecular weight of PU and various epoxy resin hardeners on dhe character of phase separation and other parameters of a hardening system was studied.