The thermal phase behavior of 1-butyl-3-methylimidazolium hexafluorophosphate, which forms three polymorphic crystals (alpha, beta, and gamma), has been re-investigated by the simultaneous measurements of Raman spectroscopy and calorimetry. The peak assigned to the phase change from beta to gamma phase is found to be exothermic and, in striking contrast with a previous report, the peak for this transition is observed near 255 K by recooling and subsequent reheating of the beta phase. This finding enabled separate measurements of the melting points (285.8 and 285.3 K), fusion enthalpies (13.1 and 22.6 kJ mol(-1)), and entropies of the beta and gamma phases, respectively. (C) 2013 Elsevier B. V. All rights reserved.