A description in thermodynamic terms is given of ligand-membrane interaction which may occur by either or both a binding and a partition process. Results obtained by fluorescence enhancement and polarization techniques on the uptake of n-(9-anthroyloxy) fatty acids by phospholipid bilayers are analysed to show that binding rather than partition effects primarily determine the extent of probe uptake. Liposome concentration-dependence effects are also reported which required that binding results obtained with different probes be compared at fixed lipid concentrations. On this basis it is concluded that as the separation of the anthracene and carboxyl groups within the fatty acid molecule is increased, and hence as the anthracene group moves deeper into the bilayer, the fluorescent probe is bound to the bilayer with greater affinity but is accepted by fewer binding sites within the membrane. Studies on probe uptake at high ionic strength and into negatively charged bilayers indicate that hydrophobic rather than electrostatic interactions make the dominant contribution to the free energy of binding.