Heat capacities of crystalline 1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide [C(4)mim][NTf2] and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C(6)mim][NTf2] in the range 80 K-T-fus were measured in an adiabatic calorimeter. Anomalies in the heat-capacity curves for the both compounds occurred near 240 K. Positions of the anomalies depended on thermal history of the samples. More stable crystals had higher heat capacities in the range 220-260 K. Below 200 K heat capacities of all the crystals of the same compound were indistinguishable. (c) 2006 Elsevier B.V. All rights reserved.