Heat capacities in a range of temperatures of (5 to 370) K, enthalpies and temperatures of phase transitions for 1-ethyl-3-methylimidazolium bis(triflamide) ([C(2)mim][NTf2]) and 1-octyl-3-methylimidazolium bis(triflamide) ([C(8)mim][NTf2]) have been determined by adiabatic calorimetry. [C(2)mim)[NTf2] has been found to form four crystalline phases with different fusion temperatures. Formation of the phases can be controlled by the temperature of annealing during crystallization. [C(8)mim][NTf2] forms three sequences of crystalline modifications, each including two polymorphs. Based on results of the measurements, thermodynamic functions for the compounds under study have been calculated. A heat-capacity anomaly near T = 230 K reported earlier for [C(4)mim][NTf2] and [C(6)mim][NTf2] have been found in some crystalline modifications of both the studied compounds. The position of the anomaly depends on the temperature of annealing of the crystals. (c) 2006 Elsevier Ltd. All rights reserved.