We found crystal polymorphism of a room-temperature ionic liquid (RTIL), 1,3-dimethylimidazolium hexafluorophosphate, by a laboratory-built calorimeter with a precise temperature controller. This polymorphism differs from hitherto reported phase behavior of other RTILs that the present RTIL lacks conformational flexibility in the associated ions. In addition, the observed difference between the melting points of two crystals, 364.3 and 314.3 K, is much larger than those reported for other polymorphic RTIL crystals. Their crystal structures were studied by Raman spectroscopy and X-ray powder diffraction. The origin of these characteristics is discussed in terms of entropy and cation-anion interactions. (C) 2011 Elsevier B.V. All rights reserved.