A quantitative study of the determination of halides and halogenic anions by negative ion electrospray mass spectrometry (ES-MS) is presented. Calibration plots were found to be linear over a range of 2.5–4 orders of magnitude for F ^- , Cl ^- , ClO ₃ ^- , ClO ₄ ^- , Br ^- , I ^- and IO ₃ ^- when a 1.0×10 ^{- 4} mol l ^-1 level of external electrolyte was added to the standard solutions. Detection limits ranged from 0.5 ng ml ^-1 for the perchlorate ion up to a blank limited 35 ng ml ^-1 for chloride. An attempt was also made to determine the level of fluoride present in mouthwash samples. Measurement of the fluoride content utilizing a calibration curve produced excellent agreement with results obtained by measurement with a fluoride ISE for two synthetic mouthwash samples but poor agreement for two commercial samples. However, excellent agreement between the two methods was obtained for the commercial samples using the method of standard additions with ES-MS.