Electron transfer is examined in three redox center (donor-bridge-acceptor) systems. Here, electron transfer cannot be described as arising from one-dimensional diffusional motion along the reaction coordinate. Instead, two-dimensional diffusion over two statistically independent reaction coordinates emerges. We show that electron transfer in three-center systems can be reduced to electron transfer between the pairs of adjacent centers. The interplay between sequential and superexchange mechanisms is examined. New expressions for the electron transfer rate, including effects of the two-dimensional reaction coordinate and of solvent dynamics, are derived. Using this analysis, new driving force effects are predicted, and rich behavior is revealed. © 1999 American Institute of Physics.