The formation of triplet states of photosensitizers and singlet oxygen during reactions sensitized by protoporphyrin IX dimethyl ester (PPDME) and the products of its photo-oxidation in solution were studied by time-resolved spectroscopy. Irradiation with long-wavelength light (670 nm), which is absorbed by the products of photo-oxidation of PPDME, provides lower quantum yields of singlet oxygen than in sensitization with PPDME alone, which absorbs light with a wavelength of 630 nm. Spectroscopic measurements also confirmed the lower rate of sensitized photo-oxidation of bilirubin during irradiation with light with a wavelength of 670 nm.