The nitrate ion reduction was studied on boron-doped diamond (BDD) electrodes by real-time on-line differential electrochemical mass spectrometry (DEMS) coupled with chronoamperomety in K+, Na+ cation-containing electrolyte solutions. It was found, via steady state voltammetry, that the hydrogen evolution reaction (HER) was affected by the presence of K+ or Na+. A moderate HER occurs in K+-containing electrolyte solution favoring the reaction between NO3- and H-2 species, whereas in Na+-containing electrolyte solutions, the HER kinetics was more important leading to a suppression of molecular nitrogen generation. The use of isotope-labeled nitrogen and DEMS confirmed the influence of alkali cations toward the nitrate ion reduction