Studies were made on the electronic absorption spectra, delayed fluorescence and phosphorescence spectra, excitation spectra of the delayed fluorescence and phosphorescence, and phase-sensitive fluorescence spectra for different concentrations of 10-formylmethylisoalloxazine (FMF) and riboflavin (RF) in the films of polyvinyl alcohol (PVA) at room (295 K) and low (103 K) temperature. It was found that the delayed fluorescence and phosphorescence of the flavins stabilized with PVA matrix was significantly affected by the relaxation effects, as well as that flavins in the excitation state formed molecular complexes (excimers or exciplexes).