The high-speed determination of hydrogen, magnesium and calcium ions by ion chromatography (IC) is demonstrated on a monolithic octadecyl-boned silica (ODS) column coated with lithium dodecylsulfate (Li-DS). This stationary phase, when used in conjunction with a 2mM ethylenediamine and 0.1mM Li-DS solution as eluent at pH 6.0, was found to be suitable for the rapid and efficient separation of hydrogen and magnesium and calcium in the order H + <Mg 2 + <Ca 2 + within 4min at a flow rate of 4.0ml/min. Under the conditions, linear calibration plots of conductivity versus concentration were obtained for the cations over about three orders of magnitude, and the detection limits were 1μM for H + , 2μM for Mg 2 + and Ca 2 + . Rainwater was analyzed directly using this IC system with satisfactory results.