The ammonium salts of three perfluoropolyether (PFPE) carboxylic acids differing in average molecular weight were examined for their phase behavior in water. Micellization was detected by surface tension measurments, whereas lyotropic liquid crystalline phases were identified by optical miscroscopy and NMR techniques. Lamellar phases have been found with shorter chain surfactants (average MW of 450 and 740) while a hexagonal phase appears to form with the surfactant with the highest MW (940). Although the composition of the three PFPE surfactant mixtures certainly contributed to determine the local microstructure, the packing features in terms of V/al were rather clearly outlined. The increase of the surfactant chain length and the addition of a cosurfactant or a cosolvent produced significant variations of the self-association structures in agreement with the expected variations of the packing parameter.