Structure, chemistry and mechanical properties of Kapton® HA and H polyimide films have been studied after their exposure to fast-neutron and gamma irradiation at a target temperature of 4 K. The fast ( E>0.1 MeV) neutron fluence was 3.1 × 10 22 n/m 2. X-ray diffraction showed that irradiation disordered partially crystalline structure of Kapton® H while the structure of Kapton® HA remained amorphous under irradiation. The glass transition temperature ( Tg) of Kapton® HA and H increased with irradiation, consistent with the crosslink formation. The increase in Tg was significantly larger in Kapton® H. The FT-IR spectrometry showed no new absorptions in the irradiated Kapton® HA and H. Shifts in some peak positions have been observed, however. Tensile testing showed that irradiation increased modulus and yield strength of Kapton® HA and H, in agreement with the proposed crosslink formation as the predominant mechanism of radiation hardening. The extent of radiation hardening, as evaluated from the total strain at fracture, was significantly larger in Kapton® H.