Abstract: he complexation capacity of water samples in Guanting reservoir waters were determined by differential pulse anodic stripping voltammetry with a double acidification method (DAM-DPASV).96 h luminescence inhibitation tests on Q67 freshwater luminescence bacterium were used to assess the toxicity of copper.Complexation capacity in Guanting reservoir was in the range of 26～128 μg/L, and in direct proportion to the DOC content.The toxicity of copper to Q67 decreased with the increase of complexation capacity.The EC₅₀ expressed as labile copper tended to be relatively constant in ranges of complexation capacity.Labile copper is therefore a better predictor of EC₅₀ than total copper in natural water.