The electroreduction of nitrate and nitrite on iridium-deposited carbon fiber electrode in aqueous sodium perchlorate solutions of pH 7 Nas studied using cyclic voltammetric techniques. Results obtained on the modified electrode support a two-step nitrate reduction mechanism in which nitrate is first reduced to nitrite at E-peak = -800 mV (Ag/AgCl) during the negative scan and further reduced to ammonia at E-peak = -775 mV (Ag/AgCl) during the anodic scan. An oxidation peak for the unreduced nitrite was observed at -300 mV (Ag/AgCl). A Tafel slope of approximately 178 mV/dec and a pseudo-first-order rate constant of k(0) = 4.18 X 10(-7) cm s(-1) were determined for the cathodic reduction peak while Tafel slope of 18.9 mV/dec and k(0) = 2.62 X 10(-23) cm s(-1) were obtained for the anodic reduction peak. The Tafel slopes obtained also support the two-step mechanism. The results obtained strongly suggest a minimal effect of pH on nitrate reduction in the pH range of 2.0-13.0 in NaClO4 solution at the surface of iridium-coated carbon fiber electrode.