Paraffins are used as phase change material (PCM) for latent heat thermal energy storage (LHTES). The efficiency of a PCM is dependent on the encapsulated quantity and energy storage capacity per unit mass during its melting and solidifying. Two different kinds of paraffin (docosane and hexacosane with melting points of 317.15 and 329.45 K, respectively) were sulfonated at three different mole percentages to increase the LHTES efficiency for this purpose. The sulfonation of paraffin samples was proven by Fourier transform infrared (FT-IR) spectroscopy. Adiabatic Bomb Calorimetry was used to determine sulfonation percentages of paraffin samples. Enthalpies of melting and solidifying of docosane, hexacosane, and their sulfonated derivatives were measured by differential scanning calorimetry (DSC) method. Thermal properties obtained by the DSC analysis indicated that the enthalpy of melting and solidifying of docosane and hexacosane could be extended by sulfonation. (C) 2006 Elsevier B.V. All rights reserved.