Thermodynamic properties of an ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim]-[PF6]) in the ideal gas state were calculated from molecular and spectral data. Because quantum chemical calculations demonstrated that K-dis for [C(4)mim] [PF6] did not exceed 10(-11) at temperatures below 1000 K, the gas was assumed to consist of ion pairs. The product of the principal moments of inertia was found to be 16.49 x 10(-132) kg(3.)m(6). The frequencies of normal vibrations were obtained from the experimental and calculated spectra. Rotation of CH3-N= was assumed to be free. The parameters for all alkyl tops were taken to be close to those in alkanes. The parameters for Bu- and PF6 were calculated ab initio. The calculated thermodynamic functions of the ideal gas (Sdegrees, C-p, and -(Gdegrees - Hdegrees(0))/T) were (657.4, 297.0, and 480.3) J(.)K(-1.)mol(-1), respectively, at 298 K and were (843.1, 424.4, and 252.8) J(.)K(-1.)mol(-1), respectively, at 500 K. Experiments were performed to better characterize the thermal stability and vapor pressure P-sat of this substance. DSC experiments were carried out in a temperature range from (303 to 523) K and suggest that the substance starts to decompose at temperatures greater than 473 K. Knudsen effusion experiments were attempted to measure P-sat for [C(4)mim] [PF6] in the temperature range (433 to 522) K, but no reproducible values Of P,at were obtained. By combining a published value of the cohesive energy density, measured heat capacities, and thermodynamic properties in the ideal gas state, thermodynamic properties of vaporization (Delta(liq)(g)C(p), Delta(vap)S, Delta(vap)H) and vapor pressure (P-sat) were calculated. At room temperature, the calculated P-sat was found to be 10(-10) Pa, a value that is much smaller than the lower detection limit for effusion measurements.