This paper presents a modified H2OCO2 equilibration method for stable oxygen isotopic composition (δ18O) analysis of water. This method enables rapid and simple δ18O analysis of milligram quantities of water, by employing solid reagent NaHCO3 as the CO2 source, a small (0.6 mL) glass vial for the equilibration chamber, and an isotope-monitoring gas chromatography/mass spectrometry (irm-GC/MS) system for analysis. This method has several advantages, including simple handling for the H2OCO2 equilibration (without purging and/or evacuation treatments), rapid and easy δ18O analysis of equilibrated CO2, and highly sensitive and highly precise δ18O analysis of H2O, using samples as small as 10 mg and with a precision of less than ±0.12‰. The time needed to attain oxygen isotopic equilibration between CO2 and water is also comparable (17 h for 10 mg H2O and 10 h for 100 mg H2O) to other previous methods using CO2 gas for the CO2 source. The extent of δ18O variation of sample water from its initial δ18O value due to isotope exchange with added NaHCO3 is also discussed. It is concluded that the correction needed is negligible (less than 0.1‰) as long as the oxygen atom ratio () is less than 3.3 ± 10−3 and provided the determination is made by comparing δ18O of CO2 equilibrated with sample water and that equilibrated with standard water of a moderately close δ18O value, less than 30‰ difference. Copyright © 2003 John Wiley & Sons, Ltd.