The dimensions of multibilayer stacks of lipids with terminal diene groups were studied before and after polymerization at variable hydration by means of X-ray diffraction, gravimetry, and infrared spectroscopy. The short-range hydration force acting between the bilayers of lipids with phosphatidylcholine and -ethanolamine headgroups is not affected by Lipid polymerization. Consequently, contributions to the interbilayer repulsion which are caused by the mobility of individual molecules are obviously negligible at distances of less than a few tenths of nanometers. The polymerized lamellae become essentially incompressible in the direction parallel to the membrane surface owing to intermolecular covalent bonds. The infrared frequencies of the methylene stretching bands of the lipids correlate linearly with the area per molecules within the membrane plane. Consequently, infrared spectroscopy in combination with the osmotic stress technique represents a convenient method to estimate lateral compressibility qualitatively. The terminal diene groups of bis(tetradecadienoyl)phosphorylethanolamine give rise to a relatively Strong component of the lateral pressure which tends to bend the Lipid monolayers into inverse structures such as the H-II phase.