Nitrite is a very important intermediate in many microbiological N transformations in soils and water. The stable isotope ¹⁵N is often used to investigate these processes. The determination of ¹⁵N in low concentrations of nitrite in the presence of large concentrations of nitrate is very difficult. Methods used so far for the isotope analysis of nitrite are unsatisfactory, because the nitrite must be calculated as the difference between nitrate plus nitrite and nitrate alone. More useful are mehods by which the nitrite is selectively converted into a chemical form that is suitable for ¹⁵N analysis and that is free from interference from other N species, particularly nitrate. Using this principle in the present study we developed a method where the nitrite is reduced to nitric oxide by iodide in acid medium. This reaction is fast and quantitative, and the ¹⁵N abundance of NO can be precisely measured by continuous flow mass spectrometry. This method is used for samples from tracer experiments with artificially enriched nitrogen 15. Therefore, the use of simple quadrupole mass spectrometers directly linked to the reaction unit is possible with sufficient precision (Reaction-Continuous Flow Quadrupole Mass Spektrometry-RCFQMS). Using the technique developed sample volumes up to 10ml containing at least 1.0 μg nitrite-N (0, 1 μg/ml) with a ¹⁵N abundance of ⩾ 0.42 at.% gave a precision of RSD ⩽ ± 3%.