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This paper presents a modified H2O CO2 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 δ^18\rmO_\rmCO_\rm2 analysis. This method has several advantages, including simple handling for the H2O CO2 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 …