A sensitive and accurate method is described for the measurement of the apparent complexing capacity of lake waters. It is based on the measurement of labile copper by differential pulse anodic stripping voltammetry after a number of ionic copper spikes have been allowed to equilibrate with the complexing materials in a water sample. Coefficients of variation for the determination of EDTA at a level of 0.50 μmole/liter in Hamilton Harbor water were 6 and 8%. Analysis of seven replicates of a Hamilton Harbor water sample gave an apparent complexing capacity of 0.52 μmole/liter Cu equivalent with a relative standard deviation of 7.7%. Analyses of a number of samples from Lake Erie and from lakes near Sudbury, Ontario gave a range from nondetectable to 0.70 μmole/liter Cu equivalent of complexing capacity.