Granular phase changing composites for thermal energy storage were made of granular porous materials and organic phase changing materials by means of vacuum impregnation method. Experimental studies on the vacuum impregnation method, phase changing behavior, chemical compatibility between porous materials and phase changing materials, and sealing performance of coating materials arrived in the following conclusions. Firstly, the vacuum impregnation method is effective in loading porous materials with phase changing materials; and its setup is simple, cheap and easy of scale-up. Secondly, organic phase changing materials (including fatty acids and their derivatives, and paraffin) and inorganic porous materials (including expanded clay, expanded fly ash and expanded perlite) are suitable raw materials for the phase changing composites with respect to chemical compatibility, large thermal energy storage density, and feasibility of large scale processing. Thirdly, thickened latex is the best choice of coating materials for the porous material granules, whose sealing performance is about 40-fold higher than that of normal cement paste and about sevenfold higher than that of the best polymer modified cement paste in this paper. © 2004 Elsevier Ltd. All rights reserved.