Raman spectroscopy has been used to study phase transitions of ionic liquids based on the triflate anion, [TfO](-), as a function of pressure or temperature. Raman spectra of ionic liquids containing the cations 1-butyl-3-methylimidazolium, [C(4)C(1)Im](+), 1-octyl-3-methylimidazolium, [C(8)C(1)Im], 1-butyl-2,3-dimethylimidazolium, [C(4)C(1)C(1)Im], and 1-butyl-1-methylpyrrolidinium, [C(4)C(1)Pyr](+), were compared. Vibrational frequencies and binding energy of ionic pairs were calculated by quantum chemistry methods. The ionic liquids [C(4)C(1)Im][TfO] and [C(4)C(1)Pyr][TfO] crystallize at 1.0 GPa when the pressure is increased in steps of similar to 0.2 GPa from the atmospheric pressure, whereas [C(8)C(1)Im][TfO] and [C(4)C(1)C(1)Im][TfO] do not crystallize up to 2.3 GPa of applied pressure. The low-frequency range of the Raman spectrum of [C(4)C(1)Im][TfO] indicates that the system undergoes glass transition, rather than crystallization, when the pressure applied on the liquid has been increased above 2.0 GPa in a single step. Strong hysteresis of spectral features (frequency shift and bandwidth) of the high-pressure crystalline phase when the pressure was released stepwise back to the atmospheric pressure has been found.