The effect of the synthetic polycation polydimethylaminoethylmethacrylate on liposome permeability was studied. Liposomes were prepared from pure dipalmitoylphosphatidyl-choline with the addition of dipalmitoylphosphatidic acid. Poly electrolyte can form electrostatic complexes with liposomes, thereby changing their permeability for liposome-entrapped 6-carboxyfluorescein. The presence of charged lipids in the liposomal membrane facilitates complex formation and increases permeability changes in pH-dependent fashion. The maximal permeability increase was observed during the first 15–30 min of poly electrolyte-liposome interaction. The mechanism of liposome permeability changes was suggested assuming charged phospholipid cluster formation on the liposome surface under the action of poly electrolyte.