Thermophysical properties were measured at ambient pressure for a high-purity sample of the ionic liquid 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C(6)mim][Tf2N], and its mixtures with water. Density was measured with two vibrating tube densimeters in the temperature range (258.15 to 373.15) K. Dynamic viscosity was determined with a rotational coaxial cylinder measuring system over the temperature range (258.15 to 373.15) K, and kinematic viscosity was measured at (293.15 and 298.15) K with an Ubbelohde capillary viscometer. Speed of sound was measured in a pulse-echo cell over the temperature range (283.15 to 343.15) K. Electrolytic (ionic) conductivity was determined by use of the AC impedance bridge technique over the temperature range (288.15 to 323.15) K. For all of these measurements, special care was taken to control the concentration of water in the sample. The viscosity and electrolytic conductivity of ([C(6)mim][Tf2N] + H2O) depend strongly on the concentration of water. By Karl Fischer titration, the H2O mass fraction in the saturated ([C(6)mim[Tf2N] + H2O) mixture was found to be 0.0111 +/- 0.0004 at 298.15 K and 0.0101 +/- 0.0004 at 293.15 K.