Several inorganic-organic hybrid complexes were synthesized from a synthetic clay (Sumecton SA) and cationic porphyrins (+4 charge). In the clay-porphyrin complexes, the lambda(max) values of the Soret bands of the porphyrins were shifted to longer wavelengths compared to those in water. Two types of complexes were formed depending on the preparation method. One is assigned to a complex in which the porphyrin molecules are adsorbed on the external surfaces of the dispersed clay layers (type b complexes). The other is assigned to a complex in which the porphyrin molecules are intercalated within the stacked clay layers (type c complexes). The aqueous solutions of both types of complexes do not scatter light in the UV-visible wavelength region. Surprisingly, the porphyrin molecules were found to adsorb on the clay sheets as densely packed monolayers with controlled intermolecular gap distance. In type b, the porphyrins are adsorbed as flat monolayers, without discernible aggregation, that precisely neutralize the negative charges of the clay surface. According to fluorescence lifetime measurements, the adsorbed porphyrin molecules have sufficiently long lifetimes to be used as sensitizers. The fluorescence lifetimes of tetrakis (AT, N N-trimethyl-anilinium-4-yl) porphyrin were found to be 4.1 ns in type b complexes and 3.2 ns in type c, while that in water is 9.3 ns. We report here a novel method in which highly dense yet controllable structures without aggregation can be produced as adsorbed layers on clay surfaces for the first time. We propose that the mechanism for this extraordinary monolayer adsorption could be a precise matching of distances between the negatively charged sites on the clay sheets and that between the positively charged sites in the porphyrin molecule. We have termed this the "size-matching effect.".