The electroreduction of 3-nitrophthalic acid anion on the mercury electrode in water + HMPA mixtures was studied with 0.5 M NaClO4 and 0.05 M NaOH as electrolytes. A detailed kinetic study concerned one-electron reduction of 3-nitrophthalic acid anion to free radical. The change of the determined rate constants of this process on water + HMPA mixture composition was interpreted in terms of the solvent adsorption on the electrode and reagent solvation. An equation is proposed which describes the observed change of the kinetics.