Photochemical energy transfer of cationic porphyrins on an "anionic -type" clay (Sumecton SA) surface was investigated. Two types of complexes were formed between the cationic porphyrin and the "anionic-type" clay depending on the conditions. With the first type, the porphyrin molecules are adsorbed on both surfaces of the individual exfoliated clay sheets or layers (type b complexes). With the second type, the porphyrin molecules are intercalated between the stacked clay sheets (type c complexes). The aqueous solutions of both types of complexes do not scatter light in the UV-vis wavelength region. For the type b complexes, we have found recently that the porphyrin molecules adsorb in a flat orientation on the clay sheets as monolayers, without discernible aggregation. The high packing density is determined by the fact that the positive charges on the porphyrin precisely neutralize the negative charges of the clay surface. Efficient photochemical energy transfer was found between porphyrins in type b complexes. Two modes of energy transfer are proposed, depending on the conditions. The first is assigned to energy transfer between porphyrins adsorbed on the same clay sheet (intrasheet energy transfer). The second is assigned to energy transfer between porphyrins adsorbed on adjacent clay sheets (intersheet energy transfer).