Divalent cation association to sonicated phospholipid liposomes has been examined with electron paramagnetic spectroscopy. Spectra were obtained suggesting that, in some cases, divalent cations associated with acidic phospholipid head groups are highly mobile. Using the amplitude of its characteristic sextet signal as a measure of free Mn(H2O)+6+, the apparent affinities of cardiolipin and phosphatidylserine for Mn2+ were measured as a function of monovalent electrolyte. Monovalent cations having smaller nonhydrated radii were more effective in displacing Mn from the phospholipids. Under conditions of low divalent cation concentrations, it is shown that the Gouy-Chapman diffuse double layer theory predicts a Mn-affinity (KA) inversely proportional to the square of monovalent salt concentration. Although this relationship was closely obeyed for Mn binding to cardiolipin, the fall-off in KA with added sodium chloride was slower in the cases of Mn binding to phosphatidylserin or phosphatidic acid. When phosphatidylcholine or cholesterol was incorporated into mixed vesicles along with a fixed amount of charged phospholipid, the Mn-binding strength was roughly proportional to the weight fraction of the latter. This result is consistent with: (1) a random dispersal of lipids in the bilayer, and (2) a 1:2 divalent cation-phospholipid interaction.