Exact expressions for the reorganization free energy for an electron transfer reaction at the liquid/liquid interface are derived using electrostatic continuum theory. All possible locations of the ions, including the cases where one or both ions are located across the interface, are considered. The resulting calculations show that allowing for ion crossing at the interface has a marked effect on the reorganization free energy, and thus on the activation free energy, for electron transfer. (C) 1998 Elsevier Science Ltd. All rights reserved.