In this article, the effects of Cl⁻ and SO₄²⁻ ions on the electrocatalytic nitrate reduction activities in a sandwich-type reactor assembly are illustrated. It was noticed that a Pt|Nafion|Pt–Cu assembly offers its best efficiency in the absence of any supporting electrolytes. The Pt–Cu thin layers adsorb chloride and sulphate ions firmly, and this adsorption blocks the H⁺ reducing sites at the cathode end of the catalytic assembly, leading to a decrease in catalytic efficiency. In the presence of chloride and sulphate ions, the reduction reactions of NO₃⁻ and NO₂⁻, respectively, are relatively favoured.