Sensitive detection of residual gases in ultrahigh vacuum is described, using nonresonant multiphoton ionization and time‐of‐flight mass spectrometry. Three‐photon ionization of Xe and Kr was successfully achieved using a focused KrF excimer laser beam. All stable isotopes of Xe were detected and the relative yields were in excellent agreement with their natural abundances. Detection sensitivity down to a partial pressure of 10−11 Torr has been demonstrated. A near‐resonant mechanism may be enhancing the Xe yield.