The tribochemical decomposition processes of ionic liquids (1-butyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium hexafluorophosphate, and 1-ethyl-3-methylimidazolium tetracyanoborate) on AISI 52100 surfaces were investigated on a friction tester in vacuum with a quadrupole mass spectrometer (Q-mass). The fluorine-containing ionic liquids showed a low friction coefficient and a small wear scar compared to fluorine-free ionic liquid. The Q-mass results indicated that ionic liquids underwent catalytic decomposition on nascent AISI 52100 surface. Increases in partial pressures derived from the cations of the fluorinated ionic liquids were detected and positively identified. This result indicated these ionic liquids decomposed by the sliding. However, the decomposition of the ionic liquids after 3 min was not identified because a reactive film was formed during this period and covered the nascent AISI 52100 surface. For the fluorine-free ionic liquid, increases in the partial pressures derived from both the cation and the anion were detected throughout the experiment, indicating that this ionic liquid did not form a reactive film on the AISI 52100 surface. An x-ray photoelectron spectroscopy (XPS) analysis revealed that fluorinated ionic liquids produced reactive films, and the type of reactive film was different for each ionic liquid. The reactive film was mainly composed of FeF2 and/or FeF3. In addition, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-butyl-3-methylimidazolium hexafluorophosphate produced reactive films comprising sulfide and phosphide, respectively.