The vacuum stability of Langmuir-Blodgett films consisting of fatty acids and Cd salts of fatty acids deposited on various substrates was investigated by means of plasma desorption mass spectrometry. Plain fatty acid films (particularly thin ones) exhibit a remarkable instability. Exceptions occur when the films are deposited on metal substrates covered with the natural oxide. In this case cation exchange occurs which results in the formation of salt (or a mixture of salt and acid) films exhibiting reasonable stability. Thin films consisting of 100% salt are fairly stable in vacuum.