A family of sterol-modified glycerophospholipids (SML) have been synthesized in which the sn-1 or sn-2 position is covalently attached to cholesterol and the alternative position contains an aliphatic chain. The liposomes formed by the SML posses a number of significant advantages over the liposomes formed from the corresponding diacyl lipid/cholesterol mixture including greatly enhanced drug retention and stability in the body. The aim of the present study is to determine the detailed molecular architecture of the monolayer formed by PChcPC, the SML containing a C16 chain and compare it with the structure of the monolayer prepared by the corresponding diacyl lipid (DPPC) and cholesterol to see how anchoring the cholesterol alters the structure of the monolayer. These studies should help explain the different in vitro stabilities of the liposomes formed by the two systems.