The objective of this work is, first, to analyze the viability of the electrochemical oxidation process by means of a boron-doped diamond electrode to treat leachates from a municipal landfill site, because a high loading of ammonium and chemical oxygen demand (COD) and poor biodegradability are the main characteristics of the leachates. The second objective of this work is to study the influence of operating conditions (i.e., applied current density, initial COD and ammonium loading, and chloride concentration) on the oxidation. Almost-complete removal of both pollutants was experimentally achieved. The kinetics of the COD disappearance was successfully predicted using a model that was based on mass-transfer control of the electrochemical process. Finally, a mathematical model that is able to describe the kinetics of ammonium removal has been reported, depending on the applied current density, the chloride concentration, and the mass-transfer coefficient of the electrochemical cell.