The miscibility and phase behavior of mixed monolayers of Ru(dpphen)(3)(2+) (dpphen = 4,7-diphenyl-1, 10-phenanthroline) (Ru(II)) and 10, 12-pentacosadiynoic acid (PDA) with different molar proportions on pure water were investigated in detail. Ru(II) is water insoluble and acts as a surface counterion. The miscibility of the two components was confirmed by the collapse pressure of the mixed monolayer and the excess free energy of mixing. It was found that Ru(II)/PDA = 1:1 is the best mixing ratio for the mixed monolayer on pure water, and any surplus Ru(II) or PDA is not favorable for the miscibility of the mixed monolayer. The dynamic elasticity and the possible condensed structure of the mixed monolayer with the optimum ratio of Ru(II)/PDA = 1:1 were discussed as well. The mixed monolayer of PDA and Ru(II) was UV-polymerized with different applied surface pressures and polymerization times. It was found that the PDA/Ru(II) molecules were isolated in small domains by the formed network during the polymerization and the dynamic elasticity of the mixed monolayer increased after polymerization. An explanation based on molecular rearrangement has been presented.