Ab initio calculations reveal that a cation, in a position equidistant from the two H-bonding oxygen atoms of (HC(OH)O · H · OH2)+, acts to push the proton from HCOOH toward OH2 if the ion is located near the —OH or C-H group of HCOOH. In contrast, rotation of the cation out of the HCOOH plane, while maintaining its position relative to the two H-bonding atoms, leads to a potential which is little different than that observed in the absence of an ion. These findings are readily explained on the basis of ion-induced internal polarization of the subunit to which the ion is closer. Replacement of OH2 by HCOOH suggests that polarization of the more distant subunit can also be a factor.