The pyrene fluorescence measurements have been carried out for various binary mixtures of micelle forming (L-alpha-diheptanoylphosphatidycholine, DHPC) and vesicle forming (L-alpha-dimyristoylphosphatidycholine, DMPC) phospholipids with different twin tail alkylammonium surfactants. The mixed micelle formation in all binary Mixtures has been evaluated and it has been observed that the mixed micelle formation between the unlike components of phospholipid and cationic surfactants takes place due to the synergistic interactions. The influence of hydrophobicity of series of twin tail cationic surfactants has been studied on the degree of synergism. (c) 2005 Elsevier Inc. All rights reserved.