The use of thin layers of water-immiscible organic solvents on electrode surfaces to evaluate rates of electron-transfer across liquid/liquid interfaces requires that reactant concentrations in the two adjacent phases be selected to minimize the effects of mass transfer on the measured currents. A general inequality is derived which provides a guide to the selection of suitable concentrations of the coreactants. Experimental tests of the inequality demonstrated its utility and should allay recently expressed concerns about the reliability of the thin-layer methodology.(11)