The relative changes in the fluorescence intensities of chlorin-p6 upon binding to phosphatidyl choline liposomes were observed to be different for one-photon (400 nm) and two-photon (800 nm) excitation at pH 5.0, 6.0 and 7.4. The corresponding fluorescence lifetimes however, were found to be similar. This is attributed to the higher two-photon absorption cross-section of the lipid bound drug. Measurements made on the changes in the lipid bound drug's fluorescence on variation of the lipid:drug molar ratio reveals that two-photon excitation might be more useful for differentiating between the free and lipid bound drug. (c) 2006 Elsevier B.V. All rights reserved.