The different mixed Langmuir monolayers of 10,12-pentacosadiynoic acid (PDA) and semifluorinated tetracosane (CF3(CF2)(7)(CH2)(15)CH3, as F8H16 below) were studied by surface pressure-area isotherm, W-Vis spectroscopy, and Brewster angle microscopy (BAM). The surface pressure-area isotherms of the mixed PDA/F8H16 monolayers show reduced limiting molecular areas, which indicate miscible films with strong interactions. A minimum limiting area was found for the monolayer of PDA/F8H16 mixed at 1:2 molar ratio. The UV-Vis spectra show that the PDA in the mixtures can be polymerized upon UV irradiation up to the molar ratio of 1:2 (PDA/F8H16). The BAM images of the mixtures show that at higher ratio of F8H16 the condensed domains of PDA disappear. The above evidence suggests that the fluorinated moiety of F8H16 may stay on top of PDA to form an extra half layer at high surface pressures.