The quantitative reduction of nitrate in an acid medium with reduced Fe was applied to the alkaline permanganate solution used to absorb NO and NO2evolved from soils during denitrification reactions. The method involves addition of H2SO4to acidify the solution and ensure oxidation of nitrite to nitrate, and treatment with reduced Fe at 100°C to reduce nitrate to ammonium. The solution is made alkaline with NaOH and ammonium determined by standard distillation procedures. It is simple and precise, and applicable to nitrogen isotope ratio analysis of NO and NO2evolved from soils.