In this study, we employed a thermodynamic method to obtain information on the second virial coefficient of the interactions between liposomes, and we also determined the stabilities of dipalmitoyl-L-alpha-phosphatidylglycerol (DPPG)- and dipalmitoyl-L-alpha-phosphatidylcholine (DPPC)-liposomes. In this study, an interaction model based on a hard-sphere interaction potential assumption was employed, and we used high-sensitivity isothermal titration nanocalorimetry (ITC) to measure the dilution heat of liposomal suspensions. Information on the second virial coefficient of interactions between liposomes can be derived from the number density of the liposomal suspension. Additionally, we qualitatively demonstrated the repulsive forces of DPPG- and DPPC-liposomes systems. In this work, both the b(2) and b(2)/B-0 levels of DPPG-liposome were greater and had better physical stability than those of DPPC-liposome. (c) 2007 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.