A gas chromatographic-mass spectrometric method, combining anO-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine–diazomethane double derivatization, was developed to identify major keto acids in ozonated fulvic acid solutions. Three keto acids, glyoxylic, pyruvic, and ketomalonic acid, four aldehydes and two analytical artifacts were identified. The detailed mass spectra, obtained in both electron impact and positive chemical ionization modes, and their fragment assignments are presented. Owing to the similarity of their electron impact mass spectra, electron impact mass spectra along with positive chemical ionization mass spectra are needed to identify these keto acids reliably. The effects of the formation of keto acids on water quality are also discussed.