The first direct measurements of the contributions of individual solvent molecules to solvent reorganization associated with photoinitiated charge transfer in a coordination complex are reported for gas-phase clusters of bis(2,2＇,2 "-terpyridyl)iron(II), [Fe(terpy)(2)](2+), with dimethyl sulfoxide (DMSO) prepared by electrospray ionization. Excitation of [Fe(terpy)(2).(DMSO)(n)](2+) clusters at wavelengths corresponding to the metal-to-ligand charge-transfer (MLCT) absorption characteristic of Fe(II)-polypyridine complexes in solution triggers evaporation of solvent molecules, permitting the application of laser photofragmentation mass spectrometry to monitor MLCT absorption. The energy of the optical transition corresponding to MLCT E-op is reported for [Fe(terpy)(2).(DMSO)(n)](2+) clusters (n = 1-11), and the cluster-size dependence of E-op is examined following Jortner＇s cluster-size-equation approach (Jortner, J. Z. Phys. D 1992, 24, 247). The first 11 DMSO molecules contribute 54-63% of the bulk solvent reorganization energy, based on estimates that E-op for the gas-phase [Fe(terpy)(2)](2+) complex falls between 1.860 x 10(4) and 1.876 x 10(4) cm(-1).