Interaction of small unilamellar vesicles (SUVs), composed of negative diphosphatidylglycerol (cardiolipin, CL(2-)) and neutral dipalmitoylphosphatidylcholine (DPPC), with poly(N-ethyl-4-vinylpyridinium bromide) (PEVP) was studied in water solution above and below the vesicular membrane melting point by means of differential scanning calorimetry, photon correlation spectroscopy, microelectrophoresis, conductometry, and fluorescence techniques. It has been found that CL(2-) species are homogeneously distributed within DPPC-CL(2-) SUV membrane leaflets and between them. Interaction of PEVP with DPPC-CL(2-) SUVs led to drastic structural rearrangements in the membrane if it was in the fluid state (liquid SUVs). Negative CL(2-) molecules migrated from the inner to the outer membrane leaflet and segregated in the vicinity of adsorbed PEVP chains. In addition, PEVP adsorption terminated completely the exchange of lipid molecules between the SUVs. At the same time, the integrity of liquid SUVs contacting PEVP remained unchanged. Since the interaction of PEVP with liquid SUVs was predominantly electrostatic in nature, the polycation could be completely removed from the vesicular membrane by addition of an excess of polyacrylic acid (PAA) polyanions forming a more stable electrostatic complex with PEVP. Removal of PEVP resulted in complete resumption of the original distribution of lipids in lateral and transmembrane directions as well as intervesicular lipid exchange. In contrast, PEVP interacting with DPPC-CL(2-) SUVs formed defects in the vesicular membrane if it was in the gel state (solid SUVs). Such interaction was contributed not only by electrostatic but most likely by hydrophobic interactions involving the defected membrane sites. PEVP kept contacting solid SUVs in the presence of an abundant amount of PAA. The established phenomena may be important for understanding the biological effects of polycations.