The J-aggregates of thiacarbocyanine dyes and naphthothiacarbocyanine dyes having bulky meso-substituent; 5,5′-dichloro-3,3′-diethyl-9-methyl-thiacarbocyanine chloride (dye I), 5,5′-dichloro-3,3′,9-triethyl-thiacarbocyanine chloride(dye II), 5,5′-dichloro-3,3′-diethyl-9-phenyl-thiacarbocyanine chloride(dye III), 3,3′-diethyl-9-methyl-4,5,4′,5′-naphthothiacarbocyanine bromide(dye IV), 3,3′,9-triethyl-4,5,4′,5′-naphthothiacarbocyanine chloride(dye V), 3,3′-dimethyl-9-phenyl-4,5,4′,5′-naphthothiacarbocyanine chloride(dye VI), in monolayer assemblies have been fabricated. From the viewpoints of steric requirement and electronic requirement proposed by us, it has been expected that mixed assemblies, donor/acceptor/arachidic acid(C 20), which incorporate two kinds of thiacarbocyanine dyes or naphthothiacarbocyanine dyes in the same layer plane, could form the homogeneous aggregate of amalgamation type (HA-aggregate), which is characterized by a single J-band and accompanying resonance fluorescence distinct from corresponding peaks of each dye. We have really observed the HA-aggregate in the combinations of dye II dye III , dye IV dye VI and dye V dye VI . On the other hand, in spite of similar molecular structure, the dye I/dye III/C 20 assembly does not form the HA-aggregate, but produces the separate aggregate or mosaic aggregate.