A method is outlined for estimating the concentration of free valinomycin (c//V) for known overall concentrations of valinomycin (c//V*) from the emf (E) developed owing to coupling of the flows of potassium and valinomycin in the membrane. This method can be used when the transport numbers of potassium and valinomycin are unity and a linear section with Nernstian slope is present in the E vs log c//V* curve. Values of c//V are estimated for film-type valinomycin membranes plasticized with dioctyl phthalate, didecyl phthalate, and dihexyl adipate.