Since shorter n-alkanes can be incorporated into lipid membranes to larger amounts, we first investigated systems with n-eicosane-d42 (C20d). 2 H NMR spectra of 10 vol% C20d in a DPPC bilayer at different temperatures are shown in Figure . Above the lipid and alkane melting temperatures (e.g. 60 • C), a spectral line shape corresponding to an anisotropic arrangement of liquid molecules is observed . This proves that the C20d molecules are indeed dispersed throughout the lipid membrane. Upon cooling below the crystallization temperature of bulk C20d (about 33 • C ), the spectrum broadens considerably. However, instead of a 2 H spectrum displaying crystal-like features, the spectrum strongly resembles the 2 H NMR spectra of phospholipid gel phases . A spectrum of perdeuterated DPPC at 30 • C, a temperature close to the DPPC gel-to-rippled phase transition , is shown in Figure for comparison. This observation indicates that C20d adopts a gel-like state at room temperature, most likely because the C20d molecules are built into the DPPC gel structure owing to their chemical similarity with the lipid acyl tails. This is reasonable since the main transition temperature of DPPC (above 42 • C due to reduced hydration levels and alkane addition ) is higher than the crystallization temperature of the alkane. Then, upon cooling, the liquid-to-gel phase transition of DPPC occurs before reaching the crystallization temperature of the alkane, and the C20d molecules become trapped in the lipid gel phase. This prevents them from crystallizing. Similar observations have previously been made for shorter n-alkanes in different lipid membranes .