Several protic ionic liquids (PILs) with a wide range of pK(a) differences (Delta pK(a)) between the parent acid and base molecules were thermally evaporated in vacuum, trapped on a CsI plate by a cryogenic neon matrix-isolation method, and studied by Fourier transform infrared spectroscopy and density functional theory calculations. The parent neutral molecules and proton-transferred cation anion pair species were identified as chemical components evaporated from the PILs with lower and higher Delta pK(a) values, respectively. The Delta pK(a)-dependent vaporization mechanism is discussed in terms of thermodynamic equilibrium between acid-base and anion-cation systems in the liquid phase.