The influence of water on the observed gas-phase population of negative ions in electrospray mass spectrometry was studied for the undiluted ionic liquid 1,3-butyl-methyl-imidazolium hexafluorophosphate (BMIM+PF6-). During the electrospray process, electrolytic reduction of water enhances the production of tetrafluorophosphate (F4PO-), which undergoes further reactions to produce difluorophosphate (F2PO2-) anions. These anions are observed in addition to the pre-existing hexafluorophosphate anion. The apparent substitution of two fluorine atoms with one oxygen is attributed to a series of reactions initiated by hydrolysis of hexafluorophosphate. This hydrolysis reaction was enhanced by the addition of hydroxide, formed via the hydrolysis of water or through the addition of ammonium hydroxide. The formation of FxPOy- was studied as a function of the electrospray current and solution flow rate. The mass spectral response shows a quantitative logarithmic relationship between SigmaFxPOy- signal intensities (adjusted for mole equivalents of H2O required) and the amount of water present, against which the water content could be rapidly assessed. Results were found to be comparable to Karl Fischer titration data.