Abstract: In this paper the correlation between different structure state of chlorophyll a (Chla) in vitro and the amounts of singlet oxygen formed has been investigated, used as a photosensitizer. Chl a of different structure state in vitro showed different ability to excite triplet molecular oxygen into singlet form. 1, 3, 5-Triphenylformazan (TF) and 1, 3-diphenyl-5-p-methoxyphenyl formazan (TF-OCH₃) was used as the quencher for singlet oxygen and phenol as indicator for oxidation products. Irradiation of the mixed system(Chl α+TF/TF-OCH₃+solv.) was carried out with a He-Ne laser (6328Å). It was found that phenol had the highest relative yield in alcohol, medium in CCl₄ and the lowest in C₆H₁₄. The results were considered to be related to the difference of structure state of Chl a in solvents of different polarity.