We investigated the dependence on solvents of optical absorption and emission of the bacteriochlorophyll a-serine (BChl-ser), a water soluble bacteriochlorophyll (BChl) derivative. Comparison between the experimental data and those collected for BChl in nonaqueous solvents shows that only a minor interaction takes place between serine and the macrocycle's π-electron system. Nevertheless, the coupling with serine results in a small enhancement of the nonradiative relaxation rate from the first excited singlet state S₁. In buffered aqueous solution (pH = 7.4), the Stokes shift of the BChl-ser fluorescence and its nonradiative relaxation rate are enhanced compared with those in nonaqueous solutions (Scherz, A., S. Katz, Y. Vakrat, V. Brumfeld, E. Gabelmann, D. Leupold, J. R. Norris, H. Scheer and Y. Salomon (1998) Photosynthesis: Mechanisms and Effects, Vol. V (Edited by G. Garab), pp. 4207–4212. Kluwer Academic, Dordrecht.), probably as a result of a hydrogen bonding between the BChl macrocycle and the water molecules. In aprotic solvents, without hydrogen bonds, the permanent dipole moment of the first excited singlet state in both BChl and BChl-ser is increased compared with the ground state by at least 2.5 Debye.