Abstract The oxidation of cholesterol and the behavior of an oxidized sterol, 5-α-hydroperoxy-cholesterol (5-AHC), have been investigated. It is demonstrated that previous work is correct in observing that cholesterol oxidation does take place at the air/water interface, but predicts initial effects and rates that are much too large. The oxidation of cholesterol is found to be autocatalytic as long as the oxidized sterol compounds (OSC) remain miscible with the cholesterol. The OSC are postulated to adopt tilted conformations with respect to the air/water interface when oxidized at or about the sterol-5,6-positions, and to segregate out when saturation OSC levels in cholesterol are reached. Pure films of 5-AHC are found to be more expanded, more compressible and less stable than those of cholesterol. In mixed films with other selected lipids, 5-AHC behaves as a greater impurity than does cholesterol when the second component is more condensed, and as a poorer condensing agent when the second component is more expanded.