Photodegradation of thin films of organic semiconducting electron-donor materials P3HT, PCDTBT, DRCN5T and BTR upon illumination with visible light in air was studied. The changes of UV–vis absorption, photoluminescence and EPR spectra were monitored as functions of illumination time. Stable radicals were generated at concentrations that exceed the values of charge carrier density of organic solar cells in the operating conditions. Increase of the radical concentration was observed for P3HT, PCDTBT, DRCN5T. On the contrary, for BTR the radical concentration drops to zero after initial increase. This indicates different photodegradation processes in studied materials.