The incorporation of a cationic polymer, such as poly(4-vinylpyridine), to the outer leaflet of negatively charged phospholipid vesicles of dimyristoylphosphatidic acid has been investigated by viscometry and intrinsic fluorescence. Viscosity changes of dilute solutions follow the size of the polymer and the polymer-liposome complex. Variations in the emission spectrum of poly(4-vinylpyridine) upon the addition of lipid vesicles under different conditions give the amount of polymer bound per mole of accessible lipid. These experimental results have been interpreted by means of the Gouy-Chapman formalism, which calculates the effective number of charges per polymer chain at the interface. The size of the adsorbed polyion has been computed from a discrete charge virial expansion that takes into account the mobility of charged groups at the interface.