Herein, the apparent equilibrium dissociation constant, K-Dapp, between Cu2+ and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-l-serine (POPS), a negatively charged phospholipid, was measured as a function of PS concentrations in supported lipid bilayers (SLBs). The results indicated that K-Dapp for Cu2+ binding to PS-containing SLBs was enhanced by a factor of 17 000 from 110 nM to 6.4 pM as the PS density in the membrane was increased from 1.0 to 20 mol %. Although Cu2+ bound bivalently to POPS at higher PS concentrations, this was not the dominant factor in increasing the binding affinity. Rather, the higher concentration of Cu2+ within the double layer above the membrane was largely responsible for the tightening. Unlike the binding of other divalent metal ions such as Ca2+ and Mg2+ to PS, Cu2+ binding does not alter the net negative charge on the membrane as the Cu(PS)(2) complex forms. As such, the Cu2+ concentration within the double layer region was greatly amplified relative to its concentration in bulk solution as the PS density was increased. This created a far larger enhancement to the apparent binding affinity than is observed by standard multivalent effects. These findings should help provide an understanding on the extent of Cu2+PS binding in cell membranes, which may be relevant to biological processes such as amyloid-beta peptide toxicity and lipid oxidation.