We have investigated the physical properties of a homologous series of synthetic, saturated 1,2-di-O-acyl-3-O-(beta-D-galactopyranosyl)-sn-glycerols using calorimetry and X-ray diffraction. Unannealed aqueous dispersions of these compounds exhibit a lower temperature, moderately energetic, chain-melting (L beta/L alpha) phase transition and a higher temperature, weakly energetic, bilayer/nonbilayer phase transition. On annealing below the L beta/L alpha phase transition, the L beta phase converts to an LC phase, which may undergo a highly energetic LC/L alpha or LC/HII phase transition at very high temperatures on reheating. The temperatures of these phase transitions are higher than those seen in the corresponding alpha- and beta-D-glucosyl diacylglycerols. However, the L beta/L alpha and bilayer/nonbilayer phase transition temperatures of the beta-D-galactosyl diacylglycerols are lower than those of the corresponding diacyl phosphatidylethanolamines. These observations are discussed in terms of the hydration and hydrogen bonding properties of their respective headgroups.