A novel amphiphilic Ru(II) complex, Ru(L18)(tPY-PO3H)PF6, where L18 = 2,6-bis(N-octadecylbenzimidazol-2-yl)pyridine and tPY-PO3H = 2,2';6',2"-terpyridine-4'-phosphonic acid, was synthesized. The complex, Ru(L18)(tPY-PO3H)PF6, at the air-water interface exhibits a molecular aggregation, which is strongly affected by the solution pH or metal coordination. Surface pressure-area isotherms of Ru(L18)(tpy-PO3H)PF6 show a smaller molecular occupied area at lower pH. Furthermore, in situ U-V-vis absorption spectra of the Ru complex at the air-water interface also depend on the subphase pH, which is distinct from the absorption spectra for CH(3)CNfbuffer or chloroform solution. As the subphase becomes acidic, the absorption band at 363 nm for the intraligand pi-pi* transitions is sharpened with a little red shift, which strongly reveals the formation of molecular aggregates at the air-water interface. The monolayers formed on different subphases can be successfully transferred onto hydrophilic and hydrophobic glass substrates. Low-angle X-ray diffractions and U-V-vis and X-ray photoelectron spectroscopy measurements indicate a regular layered structure in the Langmuir-Blodgett (LB) films transferred on the solid substrate. A tape-shaped morphology from the atomic force microscopy measurement was observed for the LB monolayer on mica. When the subphase containing metal sulfate MSO4 (M = Zn, Cd or Mn) is used, metal coordination at the air-water interface was observed. The X-ray photoelectron spectroscopy measurement demonstrates a 1: 1 ratio of the Ru complex/metal, M (M = Zn, Cd, and Mn), for the LB composite films, which is isostructural with well-characterized metal phosphonate layered solids such as Mn(O3PC6H5)H2O. The Ru(II/III) oxidative electrochemical response was observed at +0.92 V vs SCE for the monolayer LB film on the indium-tin oxide electrode, which is not varied on the metal coordination to the phosphonate group. Therefore, Ru(L18)(tpy-PO3H)PF6 is a good template at the air-water interface for the synthesis of redox-active layered inorganic composites.