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 � (298.15 to 348.15) K, dynamic viscosity η (288.2 to 348.2) K, surface tension � (298.15 to 338) K, and refractive index nD (302.95 to 332.95) K have been measured. The coefficients of thermal expansion αp 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 · min-1) 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. The chloride and water contents of ILs (as impurities) are also investigated and reported in the present work. © 2008 Elsevier Ltd. All rights reserved.