The concentration of reactants at an interface cannot be determined directly. The concentration at the interface has been estimated by measuring the interfacial tension at the interface between two immiscible solutions. An aqueous phase, de-ionised water containing penicillin G, and an organic phase containing Amberlite LA-2 were used as a typical system to study the kinetics of interfacial reaction. The interfacial tension was measured by a spinning-drop tensiometer at 25 +/- 0.1 degrees C. The interfacial tension was found to correlate well with the concentrations of penicillin G, Amberlite LA-2 and proton in each respective phase.