The molecular packing of meso-tetra( 4-N-stearylpyridyl)porphine tetra-p-toluene-toluenesulfonate (PO) was investigated at the air/water interface and in Langmuir-Blodgett (LB) films in progressively more complex systems, namely, the pure component PO, PO mixed with sodium hexadecyl sulfate (SHS), and PO mixed with SHS and stearic acid (SA). The LB films formed at selected surface pressures have been analyzed by UV-visible absorption spectroscopy. The long transition in the pure PO monolayer was ascribed to changes in the orientation of PO bases, from parallel to oblique orientation, toward the water surface. At the interface, the PO/4SHS mixture exhibits two transitions: a low surface pressure transition, attributed to a closer arrangement favoring electrostatic inter actions; and a high surface pressure transition, ascribed to the formation of an interdigitated double layer. The PO/4SHS/4SA system exhibits: the high surface pressure transition in a short range of areas. This behavior was ascribed to the coexistence of two mechanisms of long chain dense packing: the filling of PO/4SHS bases by SA molecules and the double layer interdigitation of PO/4SHS. On increasing the SA content in the ternary mixture, the transition disappears and a solid condensed monolayer, induced by the long chain packing, begins at low surface pressures.