In many lipid-containing systems (intact membranes, lipid-water and proteinlipid-water phases) the hydrocarbon chains are known to undergo a reversible temperature-dependent transition between a highly disordered (type α) and a partly ordered (type β) conformation; in the β conformation the chains, stiff and all parallel, are packed with rotational disorder according to a two-dimensional hexagonal lattice. This work describes an X-ray diffraction and freeze-fracturing electron microscope study of the phases involved in this conformational transition. Several lipid-water systems were studied: mitochondrial lipids; phosphatidic acid, synthetic lecithin; hen egg lecithin. The conformational transition is found to be a complex phenomenon dependent upon the chemical composition of the lipids, the amount of water and temperature. When the lipid is a pure chemical species the transition involves two phases; one with all the chains in the α conformation the other with all the chains in the β conformation. If the chains are heterogeneous, then from the onset of the transition from type α, they segregate into regions with different conformation, presumably according to their length and degree of saturation. One of the phases (Lαβ) consists of regularly stacked lipid lamellae, each of which is a disordered mosaic of two types of domains; one with the chains in the α, the other in the β conformation. In another phase (Lγ) each lipid lamella is formed by one monolayer of type α and one of type β, joined by their apolar faces. Two other phases (Pγ and Pαβ) display two-dimensional lattices, and consist of lipid lamellae distorted by wave-like ripples, with an ordered segregation of domains in the α and in the β conformation. The number and the structure of the phases involved in the conformational transition are strongly dependent upon the heterogeneity of the hydrocarbon chains and upon the charge and hydration of the polar groups. The results of this study have a bearing on the conformation of the chains in membranes, and on the possible biological significance of conformational transitions.