Electrocoagulation (EC) has been known for more than a century. Applications in industries such as water and wastewater treatment processes have been adapted for the removal of metals, nonmetals, suspended solids, organic compounds, COD (chemical oxygen demand), and BOD (biological oxygen demand). Iron electrodes have been preferred over aluminum due to their durability and cost. However, the electrochemical reactions occurring with EC using iron electrodes have not been systematically studied. For a better understanding of the mechanism and reactions for EC using iron electrodes, we present a review of the concept of green rust (GR) and its relationship to the current theory of EC. Experimental results obtained by measuring pH at different zones near the iron electrodes during the EC process are detailed, and are used to illustrate the mechanism and reactions that occur at both the anode and the cathode. The mechanism and reactions presented explain phenomena associated with EC and are congruent with solubility and Pourbaix diagrams.