Mixed monolayers of cholesterol and bilirubin spread at the air/water interface were used as model systems to examine the cholesterol effect on bilirubin. Miscibility and interactions between cholesterol and bilirubin were studied based on the analysis of the surface pressure-molecular area isotherms. From the isotherm data differentiated with respect to area, the condensing effect of cholesterol on the mixed monolayers could be observed distinctly. By studying surface compressibility modulus of bilirubin/cholesterol binary system vs. molecule area, we show that the liquid expanded-condensed phase transition (LE-C) of bilirubin was eliminated by cholesterol. In monolayers, bilirubin and cholesterol were found to be miscible at low surface pressure and immiscible at high surface pressure by studying the excess molecular areas of bilirubin/cholesterol system vs. mole fraction of bilirubin. The results from excess free energy of bilirubin/cholesterol system vs. mole fraction of bilirubin (X-BR) show that the maximum negative value of DeltaG(exc) appeared at X-BR=0.6, which indicates the formation of a bilirubin/cholesterol complex (MB-C) of 3:2 stoichiometry as a result of the strong hydrogen bond between the polar groups of cholesterol and bilirubin and the self-assembly characteristics of cholesterol. (C) 2004 Elsevier B.V. All rights reserved.