An experimental study on the phase change behavior of organic phase change materials (PCMs) in porous building materials is reported. Three kinds of porous materials and two kinds of PCMs were used. The phase change behavior of organic PCMs and phase change composites was measured by means of differential scanning calorimetry (DSC). The pore structure of the porous materials was characterized by means of mercury intrusion porosimetry (MIP). X-ray fluorescence spectrometry (XRF) and Fourier transformation infrared spectroscopy (FTIR) were used to characterize the chemical properties of porous materials and phase change materials. Quite different phase change behaviors were found for these two kinds of PCMs in porous materials. For capric acid with a functional group of -COOH, a remarkable elevation of melting temperature was found when confined in porous materials. But for paraffin with only inactive functional groups of -CH(2) and -CH(3), no elevation or depression of the melting temperature was found when confined in the porous materials. The interaction between functional groups of PCM molecules and alkaline spots on the inner pore surface of the porous materials and the Clapeyron equation were used to explain the different shift of the phase change temperature of capric acid and paraffin in porous materials. (c) 2006 Elsevier Ltd. All rights reserved.