The effect of dimethylsulphoxide (DMSO), a widely used solvent in life sciences to stabilise biological membrane structures at low temperatures, on the structure of model membranes has been examined using X-ray diffraction methods. It was found that increasing concentrations of DMSO in water caused a progressive decrease in lamellar repeat spacings of multilamellar dispersions of both saturated and mono-unsaturated phosphatidylcholines. The lamellar repeat spacings were temperature-dependent but with dispersions in excess 40 wt.% DMSO, repeat spacings were less than that of the phospholipids in the dry state. One dimensional electron density profiles of the lipid bilayers were calculated and the thickness of the phosphatidylcholine bilayers were determined. It was inferred from the data that the thickness of liquid-crystal bilayers decreases in the presence of DMSO and that the DMSO molecules penetrate between the polar head groups of the phosphatidylcholines, resulting in an increase in area occupied by phospholipid at the bilayer surface.