The current theory of electrocoagulation has not been able to explain all the phenomena associated with the process, among others the differences in COD removal efficiency in differing wastewaters. In this paper, we discuss the facts associated with COD removal using EC and develop a mechanism to explain them. A brief description of the technology is done and the concepts related to chemical oxygen demand (COD) are discussed prior to showing the results from a pilot plant relative to COD removal efficiency. Two set of experiments, and a mechanism for COD removal are presented. This mechanism fits both the data and observations. It follows iron Pourbaix diagram and solubility diagrams, while agreeing with the characterization of EC by-products. Finally, the factors affecting COD removal are mentioned. (c) 2007 Published by Elsevier B.V.