In situ imaging ellipsometry is employed to monitor the morphology of collapsed films of fatty acid Langmuir monolayers on pure water and on CaCl2 solution. The ellipsometry images reveal the existence of multilayer domains in the collapsed region, and analysis of the images yields the thicknesses of these domains. The multilayer films formed oil water are mainly trilayers, while those on CaCl2 solution are mainly bilayers. The structure of the collapsed films also changes sensitively depending oil the history of compression of the molecular layer.