Interfacial properties of zinc tetra(3-carboxyphenyl) porphyrin (ZnTPPC) specifically adsorbed at the water/1,2-dichlorocthane (DCE) junction were elucidated from the photoelectrochemical behavior under linearly polarized light. The photocurrent responses originating from heterogeneous quenching by ferrocene did not show a significant dependence on the penetration depth of light into the aqueous phase. However, the photoresponses exhibited a remarkable dependence on the light polarization for illumination in total internal reflection. These studies provide a rather unique insight into the correlation between interfacial reactivity and molecular orientation. The average tilting angle of the transition dipole moment appears to be directly related to the surface coverage. Finally, photocurrent responses are strongly decreased with increasing pH of the water phase. This result suggests that the self-assembly propel-ties of ZnTPPC at the water/DCE interface are linked to cooperative intermolecular hydrogen bonding involving partially protonated carboxyphenyl groups. The implication of these findings concerning solar-energy conversion as well as the molecular structure of liquid/liquid junction are briefly discussed.