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 -electron system. Nevertheless, the coupling with serine results in a small enhancement of the nonradiative relaxation rate from the first excited singlet state S 1 . In buffered aqueous solution (pH