The transition states of binary mixtures of dipalmitoyl- and dimyristoylphosphatidylcholines with sodium cholate at the reversible temperature-induced micellar-lamellar transformation were characterized by turbidimetry, electron microscopy,31P NMR and differential scanning calorimetry. This transformation is triggered by the phospholipidacyl chain melting, and appears to include two structural pathways: (i) from discoidal mixed micelles to network-like structures composed of long interlaced rod-like micelles, then to multilayer membrane structures, and finally to multilamellar vesicles; and (ii) from discoidal micelles to membrane fragments and finally to unilamellar vesicles. © 1995.