Thermodynamic functions for 1-butyl-3-methylimidazolium hexafluorophosphate ([C(4)mim][PF6]) are reported in a range of temperatures from (5 to 550) K, based on new measurements by calorimetry. Heat capacities of the crystal, glass, and liquid phases for [C(4)mim] [PF6] were measured with a pair of calorimeters. A vacuum-jacketed adiabatic calorimeter was used at temperatures between (5 and 310) K, and a heat bridge-scanning calorimeter was used from (300 to 550) K. With the adiabatic calorimeter, the fusion T-fus = 283.51 K, Delta(cr)(1)H(m)(o) = 19.60 kJ(.)mol(-1), and the glass transition T-g = 190.6 K were observed. The [C(4)mim][PF6] test sample was determined to have a mole fraction purity of 0.9956 by a fractional melting analysis. Densities of the liquid were measured in a range of temperatures from (298 to 353) K with a pycnometer equipped with a capillary neck. An unexpected endothermal transition, with a very small enthalpy change of 0.25 J(.)g(-1) (0.071 M(.)mol(-1)), was observed in a range of temperatures from (394 to 412) K. Heat capacity jumps were determined at the glass transition Delta(gl)(1)C(s) = 81.69 J(.)K(-1.)mol(-1) and fusion Delta(cr)(1)C, = 44.8 J(.)K(-1.)mol(-1), and the observed entropy change at fusion is Delta(cr)(1)S (283.51 K) = 69.23 J(.)K(-1.)mol(-1).