Ground-water samples were collected from an uncontaminated and a contaminated site. Copper complexation was characterized by ion-selective electrode (ISE), fluorescence quenching (FQ), and cathodic stripping voltammetric (CSV) titrations. All of the samples were titrated at their natural pH values and some of the samples were also titrated at other pH values. For a total Cu concentration of ten to the minus sixth M, the free Cu(2+) concentrations in the samples from the uncontaminated site were all less than ten to the minus seventh M, while free Cu(2+) in the samples from the contaminated site were all less than ten to the minus eighth M. For a particular sample and total Cu concentration, the free Cu(2+) concentration decreased as the pH increased. Relative to ISE, FQ underestimated and CSV overestimated the degree of Cu(2+) binding. The Cu(2+)-complexing properties of the ground waters are similar to many published results for the same pH and for ligand concentrations normalized to T.O.C. Chemical equilibrium computations indicate that organic complexes would dominate Cu speciation in the uncontaminated ground waters for ten to the minus seventh to ten to the minus fifth M total Cu. In the contaminated ground waters, sulfide complexes would be the predominant Cu species for total Cu less than the total S(-11) concentration. Organic complexes would dominate Cu speciation for total Cu greater than total S(-11).