Typically, the mass spectral peak observed at 19 amu in residual gas analyzers at very high vacuum (VHV) and ultrahigh vacuum (UHV) has been attributed to fluorine. Using Fourier transform mass spectrometry (FTMS), the presence of the hydronium ion H3O+ has been fully resolved from F+. Correlation of the mass 19 signals for a conventional quadrupole mass spectrometer (QMS) and the FTMS, positioned adjacent, unambiguously indicates hydronium as the source of mass 19. At partial pressures of H2O in the VHV range and higher there is sufficient density for the formation of the hydronium ion through ion-molecule interactions. Formation of the hydronium was found to directly correlate with the partial pressure of H2O. However, in a QMS, formation of H3O+ in UHV appears to occur principally by electron stimulated desorption (ESD). Dosing the system with hydrogen from 1 L exposure to saturation (1 X 10(-6) Torr for 8 h) was found to increase the H3O+ ESD yield detected by the QMS by as much as a factor of 10. The initial hydronium ESD cross section from a hydrogen saturated grid was estimated to be sigmasimilar to1 X 10(-19) cm(2). In a separate experiment, time of flight-secondary ion mass spectrometry sputter yields from the stainless steel grid of a quadrupole mass spectrometer also showed small signals of H3O+, as well as its constituents, H+, O+ and OH+, but no F+. (C) 2004 American Vacuum Society.