Under the water quality conditions in this study, NOM significantly increased the release of copper corrosion byproducts. This increase was too large to be explained by NOM-copper complex formation. NOM also decreased the electrochemically measured corrosion rate of copper. Calcium did not significantly affect either copper release or the corrosion rate. In these jar tests significantly more copper is oxidized than is solubilized without NOM. These quantitative measures of the corrosion process were supported by visual observations of the coupon surface. NOM modifies the type of scale formed on the copper surface. In the absence of NOM, the scale consisted of a discontinuous layer of coarse malachite crystals. In the presence of NOM, the scale is thinner and more homogeneous. From measurements of the Tafel slopes, it appears that NOM affects the anodic corrosion reactions more than the cathodic reactions.