Ion transfer through a liquid\liquid interface can be enhanced considerably by complexation with a suitable ionophore. Here we consider the case in which this complexation occurs at the interface. From a simple lattice-gas model we calculate the potential of mean force and the density profiles for the particles involved in the reaction. A change in the potential difference between the two phases affects the concentration of the ions at the interface, but does not change the driving force for the complexation. The rate of ion transfer depends roughly exponentially an the potential.