Thermally stable nanoporous polymer films have been created as promising materials for membrane technologies. Thermosetting Cyanate Ester Resins (CERs) and linear polyurethane (LPU) were combined by stepwise thermal curing, and the films of resulting hybrid polymers were irradiated with α-particles followed by chemical etching. Well-defined highly regular nanoporous structures were developed in the films as evidenced by using SEM. No significant changes in chemical structure of the polymer systems obtained after α-irradiation were observed by FTIR spectroscopy measurements. The polymer films obtained on the basis of hybrid networks with the CER/LPU composition equal to 90/10 wt.% had quite narrow pore diameter distribution with average pore diameters around 12 nm. DSC and TGA measurements showed that the thermal characteristics of the nanoporous films were sufficiently high (Тg ~ 167 °C to ~199 °C, Тd5% ~ 293-359 o C, and Тd max ~ 429-457 o C). The CER/LPU nanoporous thermosetting materials demonstrated effective gas transport properties tested with gases, such as О2, СО2, N2, and СН4. The combination of an additional sensitization step using -rays and longer time of chemical etching improved the properties of the nanoporous systems developed. Such materials open the way to numerous applications, for example, as ultrafiltration membranes for advanced technologies, especially under extreme conditions.