Molecular interaction is very important for the mechanical properties and application of Langmair films. In general, fatty acid film is stabilized by certain "subphase ions." In this work, two metal beta-diketonate complexes (M(tmhd)(n), tmhd = 2,2,6,6-tetramethyl-3,5-heptanedionate) were used as "surface ions" to form stable condensed films with different ratios at the air/water interface. The pi-A isotherms of the mixed films had been measured. The smaller molecular area of the metal beta-diketonate complexes indicated that the metal beta-diketonate complexes form multilayer condensed structures at high pressure at the air/water interface. However, arachidic acid (AA) retained a monolayer structure at high pressure in the mixed system. No considerable phase separations appeared during the compression of the mixed films, which indicated that the mixed films of metal beta-diketonate complexes and AA were miscible and stable. The molecular interaction of the two components in the mixed films was investigated in detail. Mixed systems with the mixing ratio of M(tmhd)(n):AA = 1:2 were chosen to study the effects of the interaction on the mechanical properties of the mixed films. The molecular interaction between AA and Ce(tmhd)(4) is proved to be more significant than that between AA and Sr(tmhd)(2), and the pi-A isotherms of the mixed films differ a lot from that of pure AA monolayer. Due to the strong intermolecular interaction, the liquid region disappeares in the Ce(tmhd)(4)/A-A mixed films, and dynamic elasticity is improved especially at high surface pressure. On the other hand, the interaction between the AA and the Sr(tmhd)(2) is much weaker and the effects of the interaction on the properties (pi-A isotherm and dynamic elasticity) of the mixed films are not so significant, especially at low surface pressure. These results are in accordant with that of the UV spectra analyses. (C) 2003 Elsevier Science (USA). All rights reserved.