From ambient- and high-pressure density measurements, we report the volume expansivities and isothermal compressibilities of four room-temperature ionic liquids and 1-methylimidazole at temperatures between 298.2 and 343.2 K and pressures to 206.9 MPa. The compounds studied are 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim][PF6]), 1-octyl-3-methylimidazolium hexafluorophosphate ([C(8)mim] [PF6]), 1-octyl-3-methylimidazolium tetrafluoroborate ([C(8)mim] [BF4]), and N-butylpyridinium tetrafluoroborate ([NBuPy] [BF4]). The density is affected by the natures of the anion, cation, and substituents on the cation, with lower densities observed for longer alkyl chains. Density decreases fairly linearly with increasing temperature but at a rate less than that for molecular organic compounds. In addition, the high-pressure measurements reveal that ionic liquids are significantly less compressible than organic solvents, with isothermal compressibility values similar to that of water. The high-pressure density data can be adequately described by the Tait equation.