Many saturated phospholipids and mixtures of saturated phospholipids have been found to form complexes with cholesterol in monolayers in the liquid state at the air-water interface. However, in these monolayers, one saturated phospholipid, dimyristoylphosphatidylcholine, shows no evidence for complex formation in mixtures with cholesterol (or dihydrocholesterol) at room temperature. Here we report clear evidence for the formation of complexes in these mixtures at 13 degreesC. At this temperature, the liquid-liquid immiscibility phase diagrams show two upper miscibility critical points with an intervening cusp at a putative stoichiometric composition. Furthermore, the chemical activity of cholesterol shows a large change at this same composition. Neither of these signatures of complex formation is observed at room temperature. The phase diagrams at the two temperatures can be modeled semi quantitatively in terms of the formation of a 3:2 phospholipid/cholesterol complex, with an exothermic heat of formation of 9 kcal/mol of phospholipid. This heat of reaction is comparable to the heat of reaction inferred from scanning calorimetry of bilayers of these mixtures.