Complexation by dissolved humic substances has an important influence on trace metal behavior in natural systems. Unfortunately, few analytical techniques are available with adequate sensitivity and selectivity to measure free metal ions reliably at the low concentrations under which they occur in natural waters. In the past, differential pulse anodic stripping voltammetry with a thin mercury film rotating disk glassy carbon electrode (DPASV-TMF-RDGCE) has been used to measure complexation capacities and conditional metal−ligand binding constants in oceanic and estuarine waters. In contrast, few studies have been conducted to validate DPASV's use in freshwaters. The current study compares DPASV and Cu ion-selective electrode (CuISE) methods for measuring free copper, complexation capacities, and conditional binding constants on samples of synthetic and isolated natural organics. Titration data show that DPASV and CuISE are able to measure similar amounts of labile copper when the two methods' detection windows overlap. Furthermore, the greater sensitivity of DPASV allows it to measure free copper at lower concentrations, which correlated well with CuISE data extrapolated into that same region. This strong correlation between these analytical methods provides positive evidence for the use of DPASV (TMF-RDGCE) in measuring free Cu2+, complexation capacities, and conditional binding constants in freshwaters.