To discuss the intervention of the ionic atmosphere on charge-transfer reactions, ab initio calculations have been carried out on the proton-transfer reaction in the (H3O2)- system, and on the fluoride-exchange reaction in the (FCH3F)- system. In a simplified model, no solvent molecules have been considered, and a very limited number of neighboring ions have been taken. Li+ and H+ cations and the positive charge have been chosen as counterions. It has been found that counterion parameters belong to the reaction coordinate, so that fluctuations of counterions may induce the chemical process. An insight to these energetic results is made from the changes in charge density produced by neighboring ions. The main conclusion is that external perturbations to the chemical system have the same effect as variations in the internal coordinates of the system, thus showing that the concept of reaction coordinate is wider than usually thought. © 1988, American Chemical Society. All rights reserved.