Conformational stabilities of 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) at high pressures have been investigated by Raman spectroscopy. The trans (planar) conformer for the CNCC angle of the [emim](+) is predominant at high pressures, but the conformational change of the CNCC angle of the [bmim](+) is independent of pressure. The gauche-trans conformer for the butyl cation of the [bmim](+) is predominant at high pressures. Thus, the difference in the alkyl chain length between the cations causes the respective conformational preferences of the two imidazolium-based ionic liquids at high pressures. (C) 2011 Elsevier B. V. All rights reserved.