The swelling pressure of two different monoglycerides, monoolein and monopalmetin, has been investigated at different temperatures through the analysis of water vapour pressure data obtained by head-space gas chromatography. The structural characteristics of the lamellar and gel phases of monoglyceride-water mixtures were determined by low-angle X-ray diffraction. In the lamellar phase, a strong repulsive force is observed., which is not very dependent on the type of monoglyceride or on the temperature. In the gel phase, the repulsive pressure is significantly lower. The range of the swelling pressure is less than for phospholipids (about 10 Å for monoglyceridews and 20-30 Å for phospholipids), demonstrating the decisive rule of the polar head-group. A comparison with the forces acting between monoglyceride layers deposited on solid surfaces, measured by a surface force apparatus, reveals a qualitative agreement bvetween the two techniques. The most significant difference is that the repulsive force at short separations (<5 Å) is lower between deposited layers than between free bilayers.