A new system for on-line analyses of metal cations has been developed by combining flow injection sample introduction with on-line capillary electrophoresis. The on-line analysis instrument has a simple construction and it allows fast sample introduction from a flowing solution. The ends of the separation capillary and the electrodes were placed opposite to each other into tubings that were installed over them behaving as flow-through channels. The capillary temperature during the analyses was controlled by water cooling and the analyte monitoring was conducted with a contactless conductivity detector. The construction allowed compound detection of very low concentrations. The instrument was used for simultaneous determination of ammonium, potassium, calcium, magnesium, and sodium as complexes in aqueous 18-crown-6 ether - acetate electrolyte solution. Flow injection analysis of sample introduction was kept apart from the ligand modified electrolyte solution. Calibration with standard mixtures of alkali metals, alkaline earth metals, and ammonium was linear in the range from 10 ng/mL to 1 mu g/mL for each analyte. The intraday reproducibility for the separations was 2% (RSD). The detection limit for the cations was 10 ng/mL. The studies showed the performance of the method to be sensitive for the monitoring of inorganic ions in water treatment.