In DMSO/water(4:1),photolysis of the dihydroxy-Sn(IV)-rnesoporphyrin dimethyl ester(SnP)/methyl viologen(MV~(2+))/ethylene diamine tetraacetic acid(EDTA)ternary system producesmethyl viologen cation radical with a quantum yield of 0.67,much higher than that of systems withother metal complexes of rnesoporphyrin dimethyl ester.Neither EDTA nor MV~(2+) quenches thestationary fluorescence of SnP,implying that the reaction does not take place at the singlet state.Withflash photolysis we obtain the T-T absorption spectrum of SnP(λ_(max)-440 nm).By following thedecay of this absorption,the triplet life time of SnP is estimated to be 41 μs.The life time is relatedto the concentration of either MV~(2+) or EDTA.Good linear relationships are obtained by plottingτ_0/τ vs.the concentration of MV~(2+) or EDTA(Stern-Volmer plot),from which we determine thequenching constants:k_q(MV~(2+))=5.5×10~7 mol~(-7) s~(-1);kq(EDTA)=2.7×10~7 mol~(-1),s~(-1).Thedata suggests that upon photolysis of the above ternary system,both oxidative quenching and reductivequenching of the triplet state of the sensitizer are occurring.From the measured phosphorescencespectrum(λ_(max) 704nm)and the ground state redox potentials (E_(1/2)~(red)～-0.84V,E_(1/2)~(ox)～Ag/AgCl,KCl(sat.)),we obtain the redox potential of triplet SnP to be E (P~+/P)～-0.33 V,E(P/P-)～+0.92 V.Matching this data with the redox potential of MV~(2+) and EDTA,we establishthe fact that during the photolysis of the SnP/MV~(2+)/EDTA ternary system,both oxidative and reductivequenching are thermodynamically favorable processes.This is also the reason why the SnP sensitizedreaction is much more efficient relative to other mesoporphyrin derivatives. Photolysis of the dihydroxy-Sn (IV) -rnesoporphyrin dimethyl ester (SnP) / methyl viologen (MV ~ (2 +)) / ethylene diamine tetraacetic acid (EDTA) radical with a quantum yield of 0.67, much higher than that of systems with other metal complexes of rnesoporphyrin dimethyl ester. Neither EDTA nor MV ~ (2+) quenches thestationary fluorescence of SnP, implying that the reaction does not take place at the singlet state. Withflash photolysis we obtain the TT absorption spectrum of SnP (λ max (max) -440 nm) .By following thedecay of this absorption, the triplet life time of SnP is estimated to be 41 μs.The life time is related to the concentration of either MV ~ 2+ or EDTA.Good linear relationships are obtained by plottingτ_0 / τ vs. the concentration of MV ~ (2+) or EDTA (Stern-Volmer plot), from which we determine thequenching constants: k_q (MV ~ (2 ) = 5.5 × 10 ~ 7 mol ~ (-7) s ~ (-1), kq (EDTA) = 2.7 × 10 ~ 7 mol ~ (-1), s ~ photolysis of the abo ve ternary system, both oxidative quenching and reductivequenching of the triplet state of the sensitizer are occurring. From the measured phosphorescence spectrum (λ max (max) 704 nm) and the ground state redox potentials (E 1/2 (red) -0.84 We obtained the redox potential of triplet SnP to be E (P ~ + / P) ~ -0.33 V, E (P ~ E / (1/2) ~ (ox) ~ Ag / AgCl, /P-)~+0.92 V. Matching this data with the redox potential of MV ~ (2+) and EDTA, we establish the fact that during the photolysis of the SnP / MV ~ (2 +) / EDTA ternary system, both oxidative and reductivequenching are thermodynamically favorable processes. this is also the reason why the SnP sensitizedreaction is much more efficient relative to other mesoporphyrin derivatives.