A new formalism was developed to investigate the lateral distribution of molecules in lipid bilayers and the results were applied to the case of phosphatidylcholine mixtures. The experimental phase diagrams for these mixtures can provide the necessary information to generate computer-simulated bilayers with the desired molecular interactions and lattice constraints. Analysis of these computer-generated lipid bilayers allows calculation of the number of contacts between like and unlike molecules, the average size and number of compositional clusters, and the pair correlation functions. A full quantitative description of the molecular organization of bilayer is provided.