To confirm that the C20d molecules are incorporated into the gel phase, we have performed X-ray scattering experiments on DPPC multi-lamellar vesicles with C20 concentrations ranging from 0 to 25 vol% at a temperature below the lipid main transition. These experiments show that the presence of C20 increases the lamellar repeat distance of the bilayers in the gel phase from 63.8±0.3 Å-1 (pure DPPC) to 72.5±1.8 Å-1 (25 vol% C20), as seen by a continuous shift of the low-q Bragg reflections to smaller reciprocal distances (Figure ). In addition, C20 affects the lateral ordering of the lipid acyl tails, as evidenced by the disappearance of the shoulder of the peak at q=1.5 Å-1 . Similar observations have previously been explained by an overall denser packing and the removal of acyl chain tilt . We therefore conclude that C20 is built into the DPPC L ′ β phase, transforming it into an L β (untilted gel) phase. The formation of the L β phase is often observed in mixtures of lipids with different acyl tail lengths , or for lipids with smaller head/tail volume ratio . Since C20 inside a bilayer of DPPC with 16-carbon long tails represents a similar system, the formation of an untilted gel in this case is not surprising.