The association of water-soluble vinyl polymers to dimyristoyl phosphatidic acid unilamellar vesicles as a function of pH, temperature and salt content using steady-state fluorescence and viscometry has been investigated. Poly(4-vinyl pyridine) fluorescence data were converted to association isotherms and discussed in terms of binding and partition models. The results of this report support previous suggestions : (1) in the case of polyions the inclusion of the activity coefficient in both models is essential; (2) the parameters calculated using the two different theoretical approaches can be directly compared by the relating equation proposed for us. Finally, the excellent agreement of steady-state fluorescence and viscometry results has allowed us to assume a model for the polymers approaching and their adsorption onto the surface, where the length of hydrophobic chain as well as the position of the nitrogen atom in the pyridinium ring play an important role.