X-ray and neutron reflectivity measurements on spread films of docosanoic acid on an aqueous subphase have been used to deduce structural information. At surface pressures of 30-40 mN m-1 the molecules are perpendicular to the subphase surface. If the subphase has pH greater than 5 and contains cadmium ions (about 2.5 × 10-4M) the ions bind tightly to the acid head groups and are not in a diffuse layer. At low surface pressures (film area twice the "close-packed" area) the film breaks up into islands (greater than 0.2 ωm in size) of close-packed molecules. At pH ≈ 6 the molecules in the islands are still perpendicular to the surface but at pH ≈ 3 they are tilted by about 30° to give a thinner film.