Rapid quantitative microchip capillary electrophoresis (CE) for online monitoring of drinking water enabling inorganic ion separation in less than 15 s is presented. Comparing cationic and anionic standards at different concentrations the analysis of cationic species resulted in non-linear calibration curves. We interpret this effect as a variation in the volume of the injected sample plug caused by changes of the electroosmotic flow (EOF) due to the strong interaction of bivalent cations with the glass surface. This explanation is supported by the observation of severe peak tailing. Conducting microchip CE analysis in a glass microchannel, optimized conditions are received for the cationic species K+, Na+, Ca2+, Mg2+ using a background electrolyte consisting of 30 mmol/L histidine and 2-(N-morpholino)ethanesulfonic acid, containing 0.5 mmol/L potassium chloride to reduce surface interaction and 4 mmol/L tartaric acid as a complexing agent resulting in a pH-value of 5.8. Applying reversed EOF co-migration for the anionic species Cl-, SO42- and HCO3- optimized separation occurs in a background electrolyte consisting of 10 mmol/L 4-(2-hydroxyethyl)-1- piperazineethanesulfonic acid (HEPES) and 10 mmol/L HEPES sodium salt, containing 0.05 mmol/L CTAB (cetyltrimethylammonium bromide) resulting in a pH-value of 7.5. The detection limits are 20 µmol/L for the monovalent cationic and anionic species and 10 µmol/L for the divalent species. These values make the method very suitable for many applications including the analysis of abundant ions in tap water as demonstrated in this paper. © 2006 Elsevier B.V. All rights reserved.