A complexation chemistry model is applied for the study of chelating ion exchange systems. A method is presented for the determination of equilibrium constants of the metal chelates with the chelating resin Dowex A-1. It is based on simultaneous pM and pH measurements in optimal pH ranges using ion selective electrodes. Equilibrium constants were determined for 1:1 calcium(II), copper(II), lead(II) and mercury(II) chelates with the resin. Theoretical distribution coefficients were calculated as function of pH for respective metal ion resin systems from the equilibrium constants determined.