The partitioning of n-alkanes between an egg-phosphatidylcholine bilayer and its torus was studied for alkanes with ten to sixteen carbon atoms using measurements of membrane capacitance. The partition coefficient was found to decrease with increasing alkane chainlength and to increase with increasing temperature. This is consistent with a well-known statistical model of lipid alkane bilayers in the liquid crystalline state. It was found that n-decane was unsuitable as a solvent in these experiments because significant partitioning of n-decane into the aqueous phase and atmosphere occurred and this could not be adequately controlled. Egg-phosphatidylcholine bilayers containing negligible amounts of solvent could be produced using a method similar to the 'freeze-out' method of White (Biochim. Biophys. Acta 356 (1974) 8-16). Bilayers formed using n-hexadecane were found to be virtually solvent-free at temperatures below 30 degrees C. The partition coefficient of n-alkanes in the bilayer was also found to depend on the alkane mole fraction. Thus it was concluded that the assumption of ideal mixing between acyl and alkane chains in the bilayer was not valid when the alkane mole fraction exceeded 40% (with respect to the acyl chains of the lipid). The variation of the standard chemical potential with temperature was measured for alkanes of different chainlengths and it was concluded that the enthalpy and entropy of the alkanes in the bilayer are in themselves temperature-dependent. This indicates that the state of the hydrophobic interior of lipid bilayers varies with temperature.