The molecular organization of a cationic matrix (DOMA) as influenced by the presence of an anionic water-soluble porphyrin (TSPP), in the mixed monolayer, molar ratio TSPP/DOMA = 1:4, has been studied by pi-A isotherms, the reflection spectra, BAM images, and imaging ellipsometry at the air-water interface. A reversible collapse of the mixed film at high surface pressures is observed. The formation of an aggregate of the porphyrin molecules under compression is inferred from the reflection spectra. BAM images show the spontaneous formation of domains at 35 mN/m that become bigger during the spontaneous dense process at constant surface pressure. The reversible character of that process was also observed by BAM: The evaluation of the reflection spectra as well as the area fraction of the domains and the surrounding dark regions have led us to consider a trilayer structure for the TSPP/DOMA domains emerging during the collapse process. By using imaging ellipsometry on selected regions of interest (few microns), the thicknesses of domains (d(dom) = 5.045 nm) and surrounding areas (d(dark) = 1.764 nm) were estimated. These thickness values are in good agreement with the trilayer architecture where the DOMA molecules have retained the TSPP molecules by electrostatic interactions.