Abstract: Sensitized photoreduction of methyl viologen produces a cation radical, MV⁺, the absorption spectrum (λ_max 605, ε/ =1.1×10⁴, λ_max395, s=3.3×10⁴) of which overlaps with that of the porphyrin sensitizer. As the MVt starts building up upon irradiation, it competes with the sensitizer for the incoming photons. Therefore, the quantum yield φ, determined experimentally decreases with irradiation time, and deviates considerably from the "real" value. Employing an expression developed by Rougee and coworkers: where f(P, MV⁺) =factor of light absorbed by P where MV⁺ is the only other species absorbing at the region.We developed a way of estimating the fraction of light absorbed by the sensitizer. All the data can be processed in an APPLE Ⅱ microprocessor. This method promised to be generally applicable to other photoreactions with the product having an absorption spectrum overlapping with the light absorbing species.