The dependence of the rate of electronic excitation transfer from a triplet donor (biacetyl) trapped inside a hemicarcerand cage to a range of triplet quenchers in free solution was studied as a function of the driving force and the internal reorganization energy of the acceptor, lambda(acceptor). Accepters with internal reorganization energies ranging from similar to 0 to more than 1.1 eV were investigated. It was found that quenchers with nearly identical triplet energies can lead to transfer rates differing by almost 3 orders of magnitude as a result of large differences in their ineternal reorganization energies. The data were analyzed in terms of the semiclassical Marcus-Jortner theory. Variable-temperature measurements were performed in order to independently evaluate the activation energies and thus to unequivocally determine which accepters belong to the "normal" and which to the "inverted" Marcus region. Four distinct groups of triplet accepters emerged from the analysis: (a) rigid aromatics with small geometry changes and modest internal reorganization energies; (b) acyclic olefins exhibiting a large-amplitude internal relaxation and correspondingly large reorganization energies; (c) cyclic olefins with exceptionally large lambda(v) values; and (d) molecular oxygen, O-2, With negligibly small internal reorganization energy.