WHILE there is an extensive literature on the methods of depositing monomolecular films of fatty substances on solid surfaces by the Langmuir–Blodgett method1, there seems to be little information available on the stability of the films once prepared. Gregg and Widdowson2 studied the evaporation of built-up multilayers of fatty acids, but their results were restricted to layers many molecules thick. More recently, Karle3 and Menter and Tabor4 have obtained some evidence of the evaporation of fatty acid and soap molecules from monomolecular films in electron diffraction investigations.