The equilibrium ion exchange method (EIM) is a powerful tool for the investigation of metal cation complexation by dissolved organic matter (DOM) in natural systems. Tests with different ion exchange resins demonstrated that under low ionic strength conditions (0.01 mol/kg) and in the presence of DOM, equilibration times of at least 24 h are required for experiments with Cu(II). The classical approach to the EIM was modified by using nonlinear reference adsorption isotherms in order to expand the method to a broader range of experimental conditions. For Cu(II) at low ionic strength (0.01 mol/kg), the reference isotherms between pH 4 and 6 were identical and were mathematically modeled in terms of Langmuir adsorption parameters. The EIM using nonlinear reference isotherms was validated between pH 4 and 6 by the correct determination of the stability constants for the complexes CuOxalate and Cu(Oxalate)₂. Then the method was used to quantitatively characterize the Cu(II) complexation behavior of DOM in an aqueous chestnut leaf litter extract between pH 4 and 6. In contrast to the classical approach to the EIM, data were analyzed by using plots [Cu]_bound/[Cu]_freevs. [Cu]_bound. This allowed the determination of both, conditional stability constants and metal binding capacities for two different binding site classes. The logarithmic values of the stability constants were about 8 for the strong binding sites and 5.5–6 for the weak binding sites. The total Cu(II) binding capacity increased from 0.22 mol/(kg C) at pH 4 to 2.85 mol/(kg C) at pH 6.