The lamellar repeat distances of aqueous dispersions of rac-1,2-dioctadec-9'-cis-enyl-glycero-3-phosphorylcholine (dietherlecithin) and 1,2-dioctade-9'-cis-enoyl-sn-glycero-3-phosphorylcholine (diesterlecithin) have been measured by X-ray diffraction as a function of water concentration. The point of maximum hydration was found to be 43% (w/w) for dietherlecithin and diesterlecithin respectively; the corresponding lamellar repeat distances being 62.3 รฅ and 60.5 รฃ. Incorporation of cholesterol above maximum hydration results in the initial increase in the lamellar repeat distance with a maximum around cholesterol concentrations of 25 and 33 mol% for dietherlecithin and die diesterlecithin respectively. The apparent partial specific volumes fo the two lecithins and for lecithin--cholesterol mixtures in sonicated aqueous dispersions were measured. Values of 1.024 cm3-g-1 and 0.987 cm3-g(-1) were obtained for diether- and diesterlecithin, respectively, at 20 degrees C. Diesterlecithin-cholesterol mixtures showed a very small change in partial specific volume while mixtures of dietherlecithin-cholesterol showed a very marked decrease with increasing proportions of cholesterol. From these data a series of structure parameters are derived for the two lecithins and possible implications for the nature of the lecithin-cholesterol interaction are discussed.