The paper presents a new method for a simultaneous determination of inorganic nitrogen species in the oxidized (NO2-, NO3-) and reduced (NH4+) form in rain water samples. The method is based on a system of nitrogen species separation employing ion exchange and diode-array detection. The ions are separated in a strong ion-exchanger, nitrites and nitrates are determined directly at 208 and 205 nm, respectively, while the ammonium ions are determined in the column hold-up time after a post-column derivatization by the Nessler reagent, at 425 nm. The use of a diode-array detector permits a simultaneous identification of the inorganic nitrogen species in 8 min. The detection limits obtained are: NO2-, 0.1 mg L(-1); NO3-, 0.05 mg L(-1); NH4+, 1 mg L(-1). The method proposed has been successfully used for speciation analysis of inorganic nitrogen in precipitation.