A sensitive assay of the²H-enrichment of water based on the isotopic exchange between the hydrogens of water and of acetone in alkaline medium is described and validated. For low²H-enrichments (0.008 to 0.5%), the sample is spiked with [U-¹³C₃]acetone and NaOH. After exchange,²H-enriched [U-¹³C₃]acetone is extracted with chloroform and assayed by gas chromatography–mass spectrometry. With some instruments, ion–molecule reactions, resulting in increased baseline enrichment, are minimized by lowering the electron ionization energy from the usual 70 to 10 eV. The²H-enrichment of water is amplified nearly sixfold in the M4/M3 ratio of [U-¹³C₃]acetone. For high²H-enrichments (0.25 to 100%), the use of unlabeled acetone suffices. After exchange, the mass isotopomer distribution of acetone is analyzed, yielding the²H-enrichment of water. The assay with [U-¹³C₃]acetone allows measuring the²H-enrichment of water even in biological samples containing acetone. This technique is more rapid and economical than the classical isotope ratio mass spectrometric assay of the enrichment of hydrogen gas derived from the reduction of water.