Discharge tubes containing argon, and NH3separated from atmospheric N2by condensation in a liquid‐nitrogen—cooled trap, are used to determine15N:14N ratios in ammonium solutions. Reduction of NH3to N2during sample preparation is not required; the dissociation of NH3in the discharge tube produces sufficiently intense N2band spectra for the isotope analysis. The method can be readily applied to the analysis of ammonium solutions derived from Kjeldahl digests of plant material.