The fluidity of lipid membranes contained in aqueous medium was studied using new fluorescent lipophilic ruthenium-ligand complexes containing dipalmitoyl-glycerophospho-ethanolamine (PE) or hexadecyl side groups, respectively. The new probes display long luminescence decay times and polarized emission, thus making them viable molecular sensors for studying the dynamics and molecular orientation of ordered liquids. Experiments are reported in which the probes were incorporated into lipid bilayers of vesicles, and their luminescence was studied with respect to temperature-dependent emission intensity, decay time and steady-state luminescence polarization. (C) 2003 Elsevier B.V. All rights reserved.