1,2-diacyl-P-O-ethylphosphatidylcholines are synthetic cationic lipids that show some promising properties as nonviral DNA transfection agents. To gain further insight in the effects of the additional ethyl group in the headgroup region on the bilayer structure we systematically investigated a homologous series of fully hydrated ethylphosphatidylcholines with linear saturated chains (C14:0, C16:0, and C18:0) by small- and wide-angle X-ray diffraction. Our data show that all of them form multilamellar vesicles with chain interdigitated gel phases. Paying regard to the very importance of the liquid-crystalline phase in gene transfection, we applied the novel MCG method on high resolution X-ray diffraction data of the C16:0 derivative to be able to gain structural information on this phase. Comparison of this ethylphosphatidylcholine with its parent compound, the unesterified phosphatidylcholine, revealed that the major difference in the liquid-crystalline phase is the significantly reduced water layer between the bilayers for the cationic lipid. This may be one factor that contributes to the comparatively good DNA transfection efficiency of ethylphosphatidylcholines.