Pyrocatechol violet (PV), a chelating agent for cupric ions was used to characterize liposomal membrane permeability. After cupric ions were added to PV liposomes, free PV turned into its chelate (PV-Cu), and encapsulated PV kept stable since liposomal membranes prevented metal ions from permeating. After the light scattering background of liposomes and the absorbance of PV were eliminated by the first-order derivative spectrophotometric method, PV-Cu i.e. free PV in liposome suspensions could be determined without separation. The released PV from liposomes could also be determined. Because PV release is relevant to liposomal membrane permeability, PV becomes a marker to characterize the membrane permeability. This new method was simple, rapid, sensitive, and was used to measure the temperature-dependent liposomal membrane permeability in this paper. Dipalmitoylphosphatidylcholine (DPPC) and soybean lecithin liposomes showed the peaks of release at 40 degrees C and 39 degrees C, respectively.