Boron-doped diamond (BDD) is a very promising material to be used in electrochemical processes of wastewater treatment, since it enables the efficient degradation of many persistent/recalcitrant organic pollutants, with the possibility of achieving their complete mineralization (i.e., combustion to CO2). It was investigated in this work the electrochemical degradation of a set of phenols frequently found in industrial wastewaters, specifically, phenol, m-cresol, catechol and guaiacol, using a BDD anode. It was the aim of this study to present a method for quantifying the extent of combustion in this type of process, by introducing the concept of "combustion efficiency", eta c, which is derived from well established theory. Experimental values of combustion efficiencies, in tests performed at different current densities were obtained for each product. The results show that the highest values of eta c are found for catechol, and that eta c increases with increasing current densities. The effect of the solute concentration on eta c was also investigated; the results show that under conditions of diffusive control eta c is independent of the concentration, pointing out that combustion and current efficiencies are independent quantities. (c) 2007 Elsevier Ltd. All rights reserved.