Cation exchange taking place in Langmuir-Blodgett films of fatty acids deposited on hydrophobic Cu substrates was investigated by plasma desorption and spontaneous desorption mass spectrometry. Complete cation exchange was found in plain arachidic acid monolayers. Incomplete exchange was found in bilayer and multilayer films of Cd arachidate with thicknesses up to 10 monolayers. The fraction of exchanged ions is independent of the number of layers.