The concentrations of formaldehyde, acetaldehyde, propionaldehyde, and n-butyraldehyde in water were quantitatively determined by head space-gas chromatography/mass spectrometry (HS-GC/MS). Aqueous aldehydes were treated with o-(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine (PFBOA) in a sealed vial at 60°C for 60 min. Head space was then analyzed by GC/MS. In comparison with electron ionization (EI) mass spectra, negative chemical ionization (NCI) mass spectra of PFBOA derivatives gave more spectral peaks of structural origin and with simple, characteristic fragmentation patterns. HS-GC/MS is highly sensitive for aqueous aldehydes because it eliminates interference from the water sample. The concentrations of various aldehydes in tap water and commercial mineral water were determined by HS-GC/MS. The following concentration ranges were detected for the individual aldehydes; formaldehyde, not detected (N.D.)-59 μg/l; acetaldehyde, N.D.-260 μg/l; propionaldehyde, N.D.-0.9 μg/l; and n-butyraldehyde, N.D.-0.3 μg/l.