In this paper the mechanism of the thermo‐and photo‐oxidation of catena‐ poly[bis(4‐benzylphenoxy)‐λ5‐phosphazene] films at short and long wavelengths has been followed under accelerated conditions using FTIR and UV‐visible spectroscopic techniques. The decrease of the band at 1453 cm−1 and the simultaneous appearance of new bands at 3400 and 3481 cm−1, due to free and bonded hydroperoxides, show that the primary hydroperoxidation in this polymer occurs at CH2 groups belonging to benzylic moietics. According to our previous paper, the bands observed in the carbonyl region at 1657, 1704 and 1725 cm−1 during photo‐oxidation have been assigned to the formation of benzophenone, benzaldehyde and benzoic acids (dimmers), respectively. The formation rate of benzophenone groups increases during irradiation at long wavelengths (λ > 400 nm). Under thermo‐oxidative condition (90°C), the benzophenone groups are the main oxidation products derived from hydroperoxide decomposition. In addition, the photooxidation of a film pre‐thermo‐oxidized for 500 h shows clearly that the benzophenone groups can generate aldehydes, benzoic acids and peracids. Crosslinking takes place in high yields by hydrogen abstraction occurring at both benzylic and secondary alcohol units. The regeneration of benzophenone groups has been evidenced during the photo‐oxidation.