In spite of the great importance of calorimetric data on phosphonium-based ionic liquids (PBILs), the information available in the literature is quite limited. This work reports the study of the thermal stability and the determination of heat capacity of the following (PBILs): tributyl(methyl)Phosphonium methyl sulfate, [(C(4))(3)PC(1)][MeSO(4)], trihexyl-tetradecylphosphonium chloride, [(C(6))(3)PC(14)][CI], trihexyl-tetradecyl-phosphonium dicyanamide, [(C(6))(3) PC(14)][DCA], trihexyl-tetradecylphosphonium bis((trifluoromethyl)sulfonyl) imide, [(C(6))(3) PC(14)][NTf(2)], and trihexyl-tetradecylphosphonium tris(pentafluoroethyl) trifluorophosphate, [(C(6))(3) PC(14)][FAP]. Measurements on the well-known IL 1-ethyl-3-methylimidazoliumbis((trifluoromethyl)sulfonyl)imide, [EMIM][NTf(2)], were also performed for comparative purposes. The thermal stability was assessed by conventional and high resolution modulated thermogravimetric analysis within the interval (303 to 873) K. The heat capacity was measured by modulated differential scanning calorimetry within the range (310 to 515)K with an uncertainty in the range (1 to 5) J . K(-1) . mol(-1). The experimental results were correlated using polynomial expressions. The Joback method for predicting ideal gas heat capacities was used in conjunction with the principle of corresponding states and the modified Lydersen-Joback-Reid method to predict the heat capacity of the ILs. The methods due to Valderrama et al. were also used with the same purpose. (C) 2011 Elsevier Ltd. All rights reserved.