The molecular-level organization of mixed and pure saturated symmetric chain 1,2-diacyl-sn-glycero-3-phosphocholines (PCs) adsorbed at a carbon tetrachloride-aqueous interface is explored by probing the hydrocarbon chain conformation within the adsorbed layer. PCs of the chain lengths found most frequently in biological systems, which in pure form are seen to form either very well-ordered or disordered layers, are observed in these studies to assemble into interfacial layers ranging from disordered to ordered states when mixed in various proportions. Independently, while C 16 and shorter chain PCs tend to form disordered layers, a strong increase in ordering is observed for C 18 and longer chain PCs in which the hydrocarbon chains are found to be primarily in an all trans conformation. Pure C 17 -PCs adsorbed at the interface produce layers with an intermediate degree of chain ordering. The ability to tune interfacial layer properties in mixed systems as a function of molecular composition, including PC chain length as demonstrated here, is an important mechanism by which surface characteristics of oil-water emulsion systems can be controlled both in vivo and in numerous commercial applications.