The passage of a phospholipid through the gel to liquid crystal phase transition is associated with an increase in the motional freedom of its fatty acyl chains as measured by spectroscopic techniques and an essentially isothermal absorption of heat as measured by differential scanning calorimetry (DSC). In addition, bilayers formed from that phospholipid display a permeability maximum for both non-electrolytes and electrolytes in the temperature region of the phase transition. In this study the sodium (and in some cases glucose) permeabilities of liposomes composed of either dimyristoyl or dipalmitoyl phosphatidylcholine plus dicetylphosphate were measured in the presence of a group of benzene and adamantane derivatives known to increase fatty acyl chain motion below the lipid transition temperature (Tc) and in the case of the adamantanes to also lower the T c as measured by DSC. None of these compounds change the temperature at which the permeability maximum occurs despite their lowering of the phospholipid T c. That is, in the presence of these additives there is observed an apparent dissociation between the phase transition and the permeability maximum. It is proposed that the permeability maximum normally observed in the temperature region of the Tc is associated with the completion of the 'melting' process. Hence a compound could cause early 'melting' of the bilayer but not change its permeability properties if the temperature at which the 'melting' process neared completion was not changed.