Abstract: Concentrations of free copper and other parameters including pH, alkalinity, IC, and TOC in the gill micro-environment of Cyprinus carpio exposed to copper of various levels under various pH conditions were measured by using Playle' s apparatus. MINTEQA2 was applied to calculate the copper speciation. Single-site complexation model was employed for characterizing the complexation capacity of the gill mucus for copper. The copper speciation in both bulk solution and the gill micro-environment was discussed.