Microencapsulation of phase change material (PCM) n-octadecane was carried out by interfacial polymerization technique using core and bulk monomers as toluene-2,4-diisocyanate (TDI) and diethylene triamine (DETA), respectively. Cyclohexane was used as the solvent for TDI and n-octadecane, which formed the oil phase. The effect of encapsulation procedure, core-to-monomer ratio (CM ratio) and PCM-to-cyclohexane (PC) ratio was investigated on core content, encapsulation efficiency, and stability of microcapsules. Using a modified procedure, the core content was found to increase with the increasing CM ratio and reached a maximum at 3.7, while the encapsulation efficiency continuously decreased with the increasing CM ratio. Also the encapsulation efficiency was found to have a strong dependence on PC ratio and a maximum encapsulation efficiency of 92%, along with the core content of 70% was obtained with CM ratio of 3.7 along with the PC ratio of 6. The microcapsules were well shaped, i.e., round and regular, with narrow size distribution at these conditions. The PCM microcapsules were found to be stable to heat treatment at 150 degrees C for 8 h. (D 2007 Wiley Periodicals, Inc.