In situ coordination of coumarin 7 (3-(2 ' -benzimidazolyl)-7-N,N-diethylaminocoumarin, CUM7), which has no substituted long alkyl chain, was found to form stable monolayers at the air/water interface with the presence of AgNO3 in the subphase. The monolayers can be transferred onto solid substrates by a Langmuir-Schaefer (LS) method. Surface pressure-area isotherms, UV-vis spectra, Fourier transform infrared spectra, X-ray photoelectron spectroscopy, and X-ray diffraction were used to characterize the monolayer formation and the transferred LS films. The LS film showed regular layer structure. Different color changes were observed for the transferred LS films upon photoirradiation with UV and visible light. The LS films showed a conductivity of ca. 0.1 S cm(-1), which is relative large for ultrathin films. It is found that although the metal complex of CUM7 and Ag(I) can be synthesized in homogeneous solutions as a precipitate, it cannot be fabricated into ultrathin films due to its insolubility in organic solvents. The in situ coordination reaction at the air/water interface makes it possible to fabricate and investigate the functional ultrathin film of this metal complex.