Abstract The interactions of iodine with oxidized cholesterol, dipalmitoyllecithin and egg lecithin were studied by the monolayer method. It was found that iodine is incorporated in the hydrophobic region of the film, unsaturated bonds being essential for the process. No interaction with hydrophilic groups could be detected. The iodide ion had no effect on the incorporation. The amount of incorporated iodine was the largest for egg lecithin, and less for oxidized cholesterol, while dipalmitoyllecithin showed no measurable effect. This tendency is in accordance with the charge-transfer complexing abilities of the lipids and the effect of iodine on the conduction of hydrated lipid samples. Our results thus favour the electronic conduction mechanism in iodine-doped bilayer systems.