Linear free energy relationships based on Abraham descriptors have been obtained for partition from water and the gas phase into hexadec-1-ene and into deca-1,9-diene, and for permeation through egg lecithin bilayers. Comparison with equations for partition into a large number of organic solvents shows that hexadec-1-ene and deca-1,9-diene are excellent models for permeation through egg lecithin bilayers. Wet octanol is a poor model, as are all polar organic solvents. The egg lecithin bilayer is non-polar and has only weak hydrogen bond acidity and weak hydrogen bond basicity. Comparison of equations for permeation through egg lecithin bilayers, through the human intestinal membrane, through the blood-brain membrane and through human skin shows that these four permeation processes are chemically far apart. It is not possible to obtain any general model for permeation: each permeation process requires a different model system.