The miscibility and phase behavior of mixed monolayers of Ru(dpphen)(3)(2+) (dpphen = 4,7-diphenyl-1,10-phenanthroline) (abbreviated as Ru(II)) and arachidic acid (AA) with different molar proportions on pure water have been investigated. Ru(II) is water insoluble and acts as a surface counterion. The miscibility of two components was confirmed by collapse pressure of the monolayer and excess free energy of mixing. It was found that Ru(II):AA = 1:2 is the best mixture ratio for the stable and condensed mixed monolayer on the pure water subphase, and any surplus Ru(II) or AA is not favorable for the mixed monolayer. The mixed Langmuir-Blodgett (LB) films with the optimum ratio of Ru(II):AA = 1:2 were deposited and characterized by UV-visible, photoinduced emission spectra, and low-angle X-ray diffraction. Both absorption and fluorescence spectra have shown that the LB films have considerable optical properties in the visible region. The possible structures of the monolayer and LB film are proposed based on the X-ray diffraction data.