Pure component pressure-volume-temperature (pVT) data for five imidazolium-based ionic liquids, 1-ethyl-3-methylimidazolium hexafluorophosphate [emim][PF(6)], 1-hexyl-3-methylimidazolium hexafluorophosphate [hmim][PF(6)], 1-octyl-3-methylimidazolium hexafluorophosphate [omim][PF(6)], 1-ethyl-3-methylimidazolium tetrafluoroborate [emim][BF(4)], and 1-hexyl-3-methylimidazolium tetrafluoroborate [hmim][BF(4)], were measured with a bellows type apparatus. High-pressure densities of ionic liquids showed smooth trends with temperature and pressure variations except for [emim][PF6] which exhibited a solid phase transition at temperatures below 352.7 K and which solidified at 100 MPa at 352.7 K. Solid densities were measured and were about 6.3 % higher than those of the liquid. Isothermal compressibilities and thermal expansion coefficients were calculated from the Tait equation that was fit to the pVT data. Isothermal compressibilities increased with alkyl chain length which is probably related to the ionic liquid free volume.