In situ microscopic measurements showed that rhombic microdomains of diprotonated 5,10,15,20-tetraphenylporphine aggregates were formed at a dodecane/aqueous H2SO4 interface. The size and thickness of the microdomains were about 10-100 mu m and a monolayer level, respectively. The microscopic absorption anisotropy of individual microdomains revealed that they had an ordered structure, like single crystals.