We examined the effect of adsorbed monovalent ions on the surface charge of phosphatidylcholine (PC) - decylamine (DA) liposomal membranes. Surface charge density values were determined from electrophoretic mobility measurements of lipid vesicles performed at various pH levels. The interaction between solution ions and the PC-DA liposomal surface was described by a six component equilibrium model. The previously determined association constants of the -PO((-)) and -N((+))(CH(3))(3) groups of PC with H(+), OH(-), Na(+) and Cl(-) ions (K (A1H), K (B1OH), K (A1Na), K (B1C1)) were used to calculate K (B2OH), and K (B2C1), the association constants of the -N((+))H(3) group of DA with OH(-) and Cl(-) ions, providing an experimental verification for the proposed model.