The surface chemistry of two new groups of surfactant carboxyporphyrins is described. These materials exhibit well-behaved monomolecular films at an air-water interface. In compressed monolayers, the porphyrin ring appears to be oriented so that the plane of the ring is perpendicular to the surface. The surface-pressure-molecular area isotherms change with the length and nature of the side chains in a manner suggesting that a long side chain allows the porphyrin rings to acquire the most ordered packing. A rigid chain structure was found to restrict the orientational flexibility of the porphyrin rings and hence prevented them from acquiring a well-ordered monolayer structure. The solid-state photophysical properties of Langmuir-Blodgett monolayers of these materials are strongly dependent on their degree of order. The absorption, fluorescence, and fluorescence excitation spectra of single monolayer films transferred onto quartz and SnO2 slides using the Langmuir-Blodgett technique indicate that-increasing the order in porphyrin monolayers leads to an increased red-shift of the Soret band and to decreased fluorescence quantum yields. On a SnO2 semiconductor surface, the monolayers exhibit enhanced fluorescence quenching. This is interpreted as evidence for isoenergetic electron transfer from the porphyrin to the semiconductor. An approximate interfacial electron-transfer rate (k(et)) was estimated on the basis of the fluorescence yields on quartz and SnO2 surfaces, respectively.