The photopolymerization of monolayers prepared at the gas-water interface with two styrene functionalized surfactants is studied and peculiar features of the film structures are described. When these films are irradiated with UV light, at constant surface pressure, their areas decrease monotonically as a function of the irradiation time. Phase separation and microscopic morphological changes induced by the polymerization process are investigated by using fluorescence microscopy and scanning force microscopy. During irradiation, domains of higher density and thickness are formed. The shapes and sizes of these domains, which are evidently the polymeric phase, depend on surface pressure and irradiation time. The film viscosities are measured before and after polymerization and are related to the irradiation time and the mean sizes of the polymeric domains. Film surface area measurements and image analysis are used to obtain the area occupied by a bound monomer in the polymeric phase. A kinetics model of the photopolymerization, taking into account the peculiar features of the almost two-dimensional system, is used to fit the experimental results of the surface area variation with the irradiation time.