Monolayers of the polymerizable 10-12 pentacosadiyonic acid amphiphile spread on a basic buffer are studied both before and after polymerization. We have established the phase diagram of the unpolymerized Langmuir film by isotherm measurements, Brewster angle microscopy, and grazing incidence X-ray diffraction experiments. Diffuse scattering experiments performed at the European Synchrotron Radiation Facility allowed us to investigate the structure and elastic properties of the different monomer phases and the modifications induced by polymerization.