The thermophysical properties of 1-hexyl-3-methyl imidazolium based hydrophobic room temperature ionic liquids （RTILs）; with tetrafluoroborate （BF4）, hexafluorophosphate （PF6）, ａnd bis（trifluoromethylsulfonyl） imide （Tf2N） anions, namely density q （298.15 to 348.15） K, dynamic viscosity g （288.2 to 348.2） K, surface tension r （298.15 to 338） K, ａnd refractive index nD （302.95 to 332.95） K have been measured. The coefficients of thermal expansion ap values were calculated from the experimental density data using an empirical correlation. The thermal stability of all ILs is also investigated at two different heating rates （10 ａnd 20） C min1） using thermogravimetric analyzer （TGA）. The experimental results presented in this study reveal that the choice of anion type shows the most significant effect on the properties of ILs. Thechloride ａnd water contents of ILs （as impurities） are also investigated ａnd reported in the present work.