The possibility that phospholipid polar heads may influence lipid peroxidation by affecting the binding and location of the metal catalyst was investigated. The multilamellar liposomes containing dimyristoyl phosphatidylcholine (DMPC) and an equal amount of either dipalmitoyl phosphatidylcholine (DPPC) or dipalmitoyl phosphatidic acid (DPPA) were utilized to study Fe3+ location. Two simple colorimetric methods were utilized, which were developed to evaluate: a) Fe3+ both sequestered within and bound in a non reducible form on the surface of the liposomes; Fe3+ chelated to strong complexing agents present both on the surface and in the inner compartment of the liposomes. The results obtained clearly show that the two types of polar heads differently bind the metal. Whereas DMPC/DPPC liposomes do not affect Fe3+ detection by both methods, suggesting that the metal is not internalized and strongly bound, DMPC/DPPA liposomes greatly interfere with Fe3+ detection. Analysis of the binding data indicates that a large amount of the metal is sequestered by the liposomes both in a complexed and free form.