The results of experimental studies of the phase transformation in composite films of the Se-Cu system obtained in vacuum as a result of the sequential thermal deposition of selenium and copper on a glass substrate are presented. At a fixed thickness of the selenium layer (45 nm), the effect of the thickness of the copper layer (40-115 nm) on the phase composition of the synthesized films formed as a result of explosive crystallization occurring during the interaction of hot copper clusters with low-melting selenium was studied. It was found that the subsequent thermal annealing of the synthesized films is accompanied by a significant change in their phase composition and optical transmission spectra. Keywords: phase transformations, composite films of the Se-Cu system, explosive crystallization, thermal annealing, optical properties.