A water soluble porphyrin, dimethoxophosphorus(V)tetraphenylporphyrin (MP(V)TPP) photosensitized folic acid decomposition in aqueous solution, resulting in the formation of a strongly fluorescent pteridine compound. The quantum yield of folic acid decomposition by photoexcited MP(V)TPP could be determined by a flurometry. A possible mechanism of the initial process of this photodecomposition is direct oxidation through an electron transfer from folic acid to the excited singlet state of MP(V)TPP. It is considered that this electron transfer proceeds as a diffusion-controlled reaction. The quantum yields of the electron transfer and the following process of folic acid decomposition could be determined. In deuterium oxide, folic acid decomposition through the photosensitized singlet oxygen generation by MP (V)TPP was also observed. A fluorometry based on the formation of a strong fluorescent compound through the oxidized decomposition of folic acid may be applied to evaluate the biomolecule damaging-activity of photosensitizers. In addition, a protocol of this assay is proposed. This simple photosensitized reaction in aqueous solution may be used for a first screening of a photosensitizer for phototherapy. (C) 2015 Elsevier B.V. All rights reserved.