Monogalactosyldiacylglycerol (MGDG) has been isolated fromVicia fabaleaves and distearoyl derivatives have been prepared by catalytic hydrogenation of the unsaturated galactolipid. The structure and the phase behaviour of 1,2-distearoyl MGDG in an excess of water were studied using a time-resolved X-ray diffraction method. The crystalline to liquid crystalline (Lc1→α) transition observed calorimetrically at 86° C was found to correspond to a lamellar crystalline to non-lamellar inverted hexagonal (H2) phase transformation. This transition appeared to be a two state process with no detectable intermediate states. In contrast cooling H2gives first Lα and then Lβ before reverting to Lc1. The transitions from the metastable lamellar gel (Lβ) phase to the lamellar liquid crystalline (Lα) state and from Lα to H2were reversible on cooling and reheating with a hysteresis of 4–5°C. A scheme consistent with the structural and calorimetric data obtained from MGDG is proposed. The similarities in the thermal and structural properties of MGDG and other non-bilayer forming lipids are discussed in terms of related functional roles of these lipid classes in biomembranes, in which they are found abundantly as natural constituents.