Mixed monolayers of a Prater-soluble tetracationic porphyrin (TMPyP) and an insoluble lipid matrix with anionic head groups (DMPA) Piers formed by the cospreading method. A ratio TMPyP:DMPA = 1:4 was selected by investigation of pi-area isotherms and stability of the cospread monolayer. Reflection spectroscopy has been used to infer the molecular organization of the porphyrin molecules in the complex monolayer at the air-water interface. A blue shift in the Soret band was observed with increasing surface pressure. The mixed monolayer of TMPyP:DMPA = 1:4 was transferred onto a glass substrate, and the molecular organization of the porphyrin was studied by transmission spectroscopy with plane polarized light (s and p) under various angles of incidence. The analysis of the reflection spectra at the air-water interface and absorption spectrum of the mixed monolayer on glass leads to a model of partial stacking of the porphyrin molecules in the monolayer at the air-water interface when the surface pressure is increased. On the glass substrate, the TMPyP molecules attached to DMPA are monomeric. Further, indirect evidence far the partial stacking model in the case of the mixture TMPyP:DMPA = 1:4 was obtained from the investigation of the mixed monolayer TMPyP:DMPA:PME = 1:4:16 (PME, methyl palmitate at the air-water interface and transferred on glass. The presence of the neutral lipid molecules facilitates dense packing of the insoluble monolayer matrix and the matching of surface charge densities of the matrix and the porphyrin moiety. In this way, the intermolecular interactions between porphyrin molecules at the air-water interface under high surface pressure are reduced and the stacking is avoided.