The electron transfer from an optically excited donor (hematoporphyrin) to an acceptor (metronidazole) has been investigated by 532nm laser flash photolysis in water, in organic solvents (alcohols, acetonitrile, formamide, N-methylformamide, N,N-dimethylformamide and dimethylsulfoxide) and in binary mixtures of these solvents with water. The electron transfer is characterized by the quenching of the donor triplet-triplet transient absorption by metronidazole. The quenching constant has a maximum value of 2.1x10 9 M - 1 s - 1 in water. This constant decreases according to an exponential function as the water content of the binary water-cosolvent mixtures decreases. In neat organic solvents, the electron transfer has only been observed in methanol and formamide, but at a rate three orders of magnitude lower than that in water. The changes in the rate constants and activation parameters of the electron transfer in going from water to neat solvents are not compatible with Marcus' theory. The water-induced electron transfer is analyzed in terms of specific solute-water interactions.