A method for the simultaneous determination of primary amino groups and ammonium ion has been proposed. The method is based in solution derivatization with o-Phthaldialdehyde/N-acetyl-cisteine (OPA/NAC) and fluorescence measurement of the formed isoindols. Analytical characteristics and description of the developed procedure have been provided. The calibration graphs for ammonium (up to 1.44mgL(-1) of N) and methylamine as primary amino model compound (up to 0.282mgL(-1) of N), were obtained. Bivariate and multivariate calibration models have been tested. The limits of detection were 0.07mgL(-1) of N and 0.004mgL(-1) of N for ammonium and amine, respectively. The procedure was first applied directly to standard solutions containing ammonium and amines and secondly to digested solutions by Kjeldahl method. The results obtained allowed to establish the best digestion conditions in order to perform the total amine conversion into ammonium. This procedure has been also applied to real samples (irrigation ditch, residual and fountain waters) and the concentrations of primary amine groups and ammonium have been evaluated. The results obtained after Kjeldahl digestion allowed to estimate the total Kjeldahl N contained in the samples. The samples were also analysed by Nessler method and similar results were obtained.