The aggregation of a water-soluble anionic porphyrin promoted with a cationic amphiphile at the liquid/liquid interface was observed for the first time by means of a second harmonic generation-linear dichroism (SHG-LD) and SHG-circular dichroism (SHG-CD). The linear polarization rotation analysis of the SHG intensity and the observed SHG-CD spectra elucidated the generation of a characteristic optical activity accompanied by the formation of the interfacial aggregate of inherently achiral porphyrin molecules. (C) 2004 Published by Elsevier B.V.