To monitor the biological action at the membrane level induced by photosensitizing agents, we studied the thermodynamic changes in the phase transition temperature(T), enthalpy(ΔH), entropy(ΔS), and H(height)/HHW(half-height width)of dimyristoylphosphatidylcholine liposomes induced by camphorquinone(CQ), benzil(BZ), 9-fluorenone(9F), benzoin methyl ether, azobisisobutyronitrile, acridine orange(AO), anthracene(AN), and neutral red(NR)using differential scanning calorimetry.For photoinitiators 9F+DM(N, N-dimethylaminoethyl methacrylate)and BZ+DM caused large changes in the T and ΔH(T shift to a low temperature)and a large decrease in the ΔH, whereas the changes of CQ+DM were smaller than those of the 9F and BZ systems.Action of aromatic ketones(9F, BZ)to liposomes was higher than that of aliphatic ketones(CQ).The T of AO and NR, which was a small decrease in the ΔH, was shifted to a higher temperature(1〜3℃).Also, AN, AO, NR caused large changes in the ΔH and ΔS at 30℃, pH10 when oxygen was present in their systems during photosensitization.Since DMPC does not have double bonds in its molecules, lipid peroxidation does not occur.Therefore, it is suggested that the large interaction of photosensitizing agents with liposomes may be due to the excited charge-transfer complexes between photosensitizer(CQ, BZ and 9F)and reducing agents(DM), and between photosensitizing agents(AN, AO, NR)and oxygen.