Monolayers of fatty acids and other long-chain polar compounds adsorbed by retraction on to the surface of freshly evaporated metal films have been studied by electron diffraction. The use of transmitted electrons has enabled the inter-molecular spacing to be measured. Although monolayers have no definite crystalline structure, the molecular arrangement in fatty acid monolayers on unreactive metals is not completely random. The end-groups appear to be packed in much the same way as they are in one of the polymorphic forms of the bulk crystal, but with the hydrocarbon chains perpendicular to the basal plane instead of inclined as in the bulk crystal. This regularity of packing only extends for short distances and is destroyed if the temperature is raised, or if reaction of the end-group with the underlying surface can occur. Reaction with the surface or a reduction in the size of the end-group allows closer packing of the molecules.