An evaluation was made of four models for determining apparent stability constants of Cu(II) complexes with humic acids (HA), using experimental data for both the amounts of Cu(II) bound to the complexes (ISE measurements), and for protons released from reactive sites (base titrations). A continuous distribution model based on the Scatchard plot showed the greatest promise for modeling the binding data. Constants obtained in this way are considered to reflect variations in binding energies without regard to the manner in which Cu(II) is bound. Binding affinity of a soil HA for Cu(II) was affected by pH, ionic strength (I), and, at pH 4, the concentration of HA. Values for log Kintat pH 4, 4.5, and 5 (I of 0.005 M) were 7.62, 7.80, and 8.50, respectively. At pH 4, log Kintdecreased with increasing I, absolute values being 7.62, 7.10, 7.00, and 6.81 at I values of 0.005, 0.025, 0.05, and 0.1, respectively. Results of base titrations showed that, at pH 4, an average of 1.45 protons were released per molecule of Cu(II) bound, indicating the formation of 1:1 and 2:1 complexes (i.e., CuL−and CuL2, where L is the reactive site of the macromolecule). Complexation of Cu(II) by HA can best be explained on the basis of the formation of polynuclear complexes.