The structure of the mixed Langmuir and Langmuir-Blodgett (LB) films of arachidic acid (AA) and perfluorotetradecanoic acid (PA) are investigated. Surface pressure-area isotherms of the mixed Langmuir films of AA and PA fall between those of pure AA and pure PA, indicating that the additivity rule holds in this system. This shows that the two components are phase-separated or are ideally mixed. The results of transmission and reflection-absorption IR spectroscopic measurements of the mixed LB films reveal that the composition of the LB films is the same with that in the spreading solution and that the orientation of the two components does not change with mixing. Molecular axes of AA and PA are almost perpendicular to the film surface. AFM and FFM show that AA forms distinct monolayer domains that are surrounded by monolayer PA. These results indicate that AA and PA are phase-separated in the mixed LB films. On the other hand, rather different results were reported for the mixed Langmuir and LB films of AA and PA in the literature, where the additivity rule did not hold for the surface pressure-area isotherms and barely discernible domains were observed in the AFM images. The different behaviors should be due to the differences in the experimental conditions such as the spreading solvent.