A novel amphiphilic Ru complex, [Ru(L18)(pdc)], was synthesized, where L18 = 2,6-bis(2'-(1'-octadecylbenzimidazolyl))pyridine and pdc = 2,6-pyridinedicarboxylate. When this complex was spread on the air-water interface as a Langmuir-Blodgett (LB) monolayer, the chemical reaction at the interface with water occurred. The resulting product at the interface could be transferred on glass or indium-tin oxide (ITO) substrate as a stable LB film. The in-situ UV spectra indicates the MLCT band at 500 nm was shifted to a longer wavelength (680 nm). The v(CO) stretching of [Ru(L18)(pdc)] was changed from 1646 cm(-1) to 1735 cm(-1). These results strongly suggest that the coordinated carboxylate groups were protonated at the air-water interface to obtain the [Ru(L18)(py(COOH)(2))(OH)(2)].