The two isomeric compounds, octadecylfumaric acid and octadecylmaleic acid (C 18 H 37 OCOCH=CHCOOH) have been synthesized, and Langmuir-Blodgett films of their cadmium salts have been studied by means of transmission and grazing incidence reflection infrared spectroscopy. These measurements provided information about the orientation of molecular segments relative to the substrate surface as well as about the lateral packing of the chains in the unit cell. A structural change of the maleate film was revealed at elevated temperatures. Polymerization of the films was brought about by either UV irradiation or thermal treatment and was followed spectroscopically as a function of exposure time. By comparison with films polymerized on the water surface of a Langmuir trough, it was concluded that the molecular structure is independent of where the polymerization occurs. Interestingly, upon polymerization on the water surface the fumarate undergoes an expansion at constant surface pressure whereas the maleate contracts. A mixture of fumarate and maleate in the ratio of 1 to 3, however, forms a two dimensional solution and preserves its area upon polymerization, a critical feature required for the fabrication of crack free Langmuir-Blodgett films polymerized on solid substrates. © 1986 American Institute of Physics.