Abstract The formation of the anion radical of merocyanine 540 (MC − ) and its properties in methanol-aqueous media were studied using electron spin resonance (ESR), pulse radiolysis (PR) and laser flash photolysis (LFP). On illumination of MC, in the presence of ascorbate or catechol, characteristic ESR signals were observed due to ascorbyl or o -semiquinone radicals respectively. The durosemiquinone radical was detected in the illuminated samples that contained MC, reduced nicotinamide adenine dinucleotide (NADH) and duroquinone. The nature of the excited state of MC responsible for such electron transfer reactions was determined using LFP experiments. The LFP data indicated rapid quenching of the triplet excited state of MC with concurrent formation of MC − in the presence of ascorbate. In PR experiments, MC − was generated by an efficient interaction of the solvated electron with MC ( k = 6 × 10 9 M −1 s −1 ). It was found that MC − had an absorption maximum at 400 nm with ϵ = 5.9 × 10 4 M −1 cm −1 . In the absence of electron acceptors, MC − decayed probably via disproportionation. In the presence of oxygen or ferric ions (as in the complex with diethylenetriaminepentaacetic acid (DTPA)), the decay of MC − was greatly enhanced indicating efficient interaction between these species ( k = 1.7 × 10 9 M −1 s −1 and k = 6.5 × 10 8 M −1 s −1 for the reaction of MC − with O 2 and DTPA-Fe 111 respectively). Hence photoinduced electron transfer and the formation of MC − which may occur in MC-photosensitized systems could lead to the generation of oxygen radicals. The formation of hydroxyl radicals in such systems was shown indirectly by ESR spin trapping of methanol-derived radicals. It is postulated that the hydroxyl radicals are generated via a Fenton reaction driven by reducing radicals derived from photoinduced electron transfer reactions. An additional mechanism for the photodynamic efficiency of MC is proposed based on electron transfer reactions induced by light.