The mechanisms of nitrate and nitrite ion electroreduction on a carbon fiber electrode modified with platinum group metal catalyst are studied. Chronoamperometry studies reveal that two different mechanisms exist in two cathodic regions. A plot of log i vs. E reveals the presence of adsorption pseudocapacitance and is supported by impedance measurement at the electrode/solution interface. In the -600 to -800 mV (Ag/AgCl) potential region, the Tafel slope for nitrate ion reduction varies between cu. 221 and 236 mV/dec. while in the -925 to -1000 mV (Ag/AgCl) region, the slope was ca. 80 to 85 mV/dec. For nitrite ion reduction, the Tafel slope was ca. 232 to 250 mV/dec in the -500 to -700 mV (Ag/AgCl) potential region, and between -925 and -1000 mV (Ag/AgCl), the slope was between 80 and 86 mV/dec. The adsorption of nitrogen dioxide on the electrode surface was determined to be the rate-limiting step in both potential regions. The present results indicate that although the first step of nitrate reduction in neutral solution takes a similar path as nitrate/nitrite ion reduction in alkaline solution, the rest of the mechanistic pathways are different in alkaline or acidic solution.