If C30d inside DMPC membranes crystallizes at only slightly lower temperatures than bulk C30d, the anisotropic liquid C30d component which is observed between approx. 45 and 60 • C in the 2 H spectra must originate from n-alkane molecules inside the membrane which do not crystallize. We suspect that these non-crystallizing n-alkane molecules are build into the DMPC phase, similar to C20 in DMPC. Such a co-association of alkane and lipid is supported by the increased main transition temperature of DMPC in the presence of C30d. As mentioned previously, a shift of the lipid main transition temperature is expected when adding longer n-alkanes to saturated phospholipid bilayers. The loss of the pretransition temperature has also been observed previously in mixtures of C12 or C14 with DPPC . It should however be noted that, while the DMPC and C30d/DMPC samples compared in figure were prepared in an identical manner, hydration somewhat differs between the samples. After preparation, the samples contained 22.5 and 12.5 water molecules per lipid for DMPC and C30d/DMPC, respectively. Since lipid acyl chain ordering increases with decreasing water content below approx. 25-30 water molecules per lipid , the shift of the DMPC main transition partially relates to a decrease in hydration. However, at the estimated water concentration, we would not expect such a large shift of the main transition temperature, and the pretransition should still exist (in the absence of alkane) . Therefore, we are convinced that the shift of the lipid main transition demon-Temperature / °C strates an association between alkane and lipid molecules.