The bimolecular rate constants for triplet energy transfer between biacetyl (2,3-butanedione) encapsulated within a hemicarcerand and a series of triplet energy accepters are reported. Because all of the rate constants for energy transfer involving 1 . biacetyl are well below the diffusion-limited value, they can be interpreted as reflecting differences in the rates of energy transfer. There is a significant spread in the rate constants of energy transfer for 1 . biacetyl with the slowest acceptor, molecular oxygen, 3 orders of magnitude slower than the fastest acceptor, pyrene. The large decrease in the values of the rate constants can be accounted for by a model in which the increase in intervening distance decreases the electronic coupling between the donor-acceptor pair. The variation in energy transfer rate constants can be explained by a Marcus dependence on the thermodynamic driving force that places exothermic energy transfer into the inverted region.