The effectiveness of the decontamination of waters by means of electrochemical technologies depends on the different operating conditions of the processes. However, chemical composition of the effluents can be also a determining parameter for removing organic pollutants. In this frame, the water hardness must be considered as an important condition to be studied because it varies drastically in waters. Therefore, electro-Fenton (EF) efficacy was evaluated to treat synthetic effluents with different hardness in presence of model organic pollutants (azo dyes, Ericochrome Black T (EBT) and Methyl Orange (MO)) by using a stirred boron-doped diamond/air-diffusion electrochemical cell. Results clearly demonstrated that the electrochemical decolorization kinetics of MO is not affected by the content of Mg2+ or Ca2+ in solution. Conversely, the decolorization of EBT is considerably reduced when an increase on the water hardness is achieved (increasing Mg2+ or Ca2+ concentrations in solution). This behavior is given due to the formation of calcium and magnesium complexes with EBT by limiting the electrophilic attack of (OH)-O-center dot radicals. Meanwhile, MO is not complexed by these metallic centers favoring its elimination from synthetic effluent by EF technology. Another interesting feature is that chemical oxygen demand (COD) was reduced up to 25% when the water hardness was increased from 0 up to 360 mg L-1 of CaCO3 in solution for EBT. This suggests that the formation of calcium or magnesium organo-complexes enhances the stability of the pollutants and as a consequence reduces the overall mineralization efficiency. (C) 2016 Elsevier Ltd. All rights reserved.