Naphthol, a chemical classified as an endocrine disruptor and known to show carcinogenic activity, is found in the environment as a mixture of its isomers. For the first time, the truly simultaneous voltammetric determination of these isomeric forms of naphthol was explored in this work using a cathodically pretreated BDD electrode (CPT-BDD). The isomers 1-naphthol (1-NAP) and 2-naphthol (2-NAP) showed an irreversible oxidation process at 0.80 V and 0.96 V vs. Ag/AgCl (3.0 mol L-1 KCl), respectively, which allowed the simultaneous determination of these compounds. The electrochemical features of the 1-NAP and 2-NAP electrooxidation on CPT-BDD were investigated, including the determination of apparent diffusion coefficients and adsorption studies. Using differential pulse voltammetry under optimized experimental conditions, the analytical curves simultaneously constructed for 1-NAP and 2-NAP were both linear in the concentration range of 0.200 to 3.85 mu mol L-1 with limits of detection of 50 and 100 nmol L-1, respectively. Because the naphthol isomers do not adsorb on CPT-BDD, an excellent repeatability of measurement was verified and the proposed electroanalytical procedure was successfully applied for a truly simultaneous quantification of 1-NAP and 2-NAP in water and synthetic urine samples. (C) 2017 Elsevier Ltd. All rights reserved.