The organization of a cationic water-soluble porphyrin, TMPyP, in a complex monolayer containing an anionic phospholipid, DMPA, at the air-water interface has been directly inferred by Brewster angle microscopy, BAM. In a previous work (Martin, M. T.; Prieto, I.; Camacho,L.; Mobius, D. Langmuir 1996, 12, 6554) the organization of this mixed monolayer TMPyP/DMPA, in the ratio 1:4, was explained on the basis of a model where the TMPyP molecules are in a monomer-dimer equilibrium depending on the surface pressure, that is, on the surface density of the porphyrin, by analysis of pi-A isotherms and reflection spectra results. The morphology of the mixed monolayer was recorded at several surface pressures. The images are different from those obtained for the lipid matrix. At 5 mN/m a homogeneous and continuous film was observed. However, for surface pressures > 8 mN/m the coexistence of two phases is recorded, and small domains with higher brightness than that of the surrounding area are formed. The density of the bright domains increased until covering nearly the whole surface at 35 mN/m. The BAM results have been related with those obtained by reflection spectroscopy at the air-water interface. The appearance of the bright domains has been attributed to the formation of the dimer phase II where the DNPA molecules are dense-packed with an areaA(DMPA,II) = 0.40 nm(2), and the homogeneous area has been attributed to phase I with porphyrin monomers. In phase I, the DMPA molecules are in a liquid-expanded state with the area A(DMPA,I) = 0.84 nm(2).