The interactions of a cationic polyelectrolyte, poly(vinylbenzyltrimethylammonium chloride) (PVTA), with anionic lipid bilayer membranes composed of a mixture of phosphatidylglycerol (POPG) plus phosphatidylcholine (POPC) have been examined using H-2 NMR of choline-deuterated POPC to probe membrane surface electrostatics. PVTA produces distinct membrane domains containing a 1:1 PVTA/POPG cation/anion charge ratio. The remaining PVTA-free domains are depleted with respect to POPG.