The parameters affecting the vacuum distillation method for quantitative separation of ammonium and nitrate for mass spectrometric determination of15N were investigated. The vacuum distillation was found to be advantageous to steam distillation method because no interference from organic compounds was encountered, greater volumes could be processed and no previous evaporation of the water samples was necessary. The mass spectrometric isotopic analysis of nitrogen 15 proved the suitability of the vacuum distillation method for quantitative separation of ammonium and nitrate.