Two-dimensional (213) molecular films composed of mixtures of an amphiphilic polymer containing an azobenzene (Az) side chain [6Azl0-PVA; PVA = poly(vinyl alcohol)] and a nematic liquid crystal molecule, 4'-pentyl-4-cyanobiphenyl (5CB), are prepared on both the water surface and solid substrate at various molar ratios. The compatibility and phase separation behavior of the two components in two dimensions are examined. Surface pressure area isotherms and atomic force microscopy shows that 2D phase separation starts to occur on a submicrometer scale above the critical ratio of 5CB to Az units (r = 2), while the two components are homogeneously mixed at a molecular level and form a hybrid monolayer up to the critical ratio. It is further revealed that phase separation is composed of projecting thicker domains of the 6Az10-PVA/5CB molecular hybrid ([Az]/[5CB] = 1:2) and pure 5CB continuous phase. Photochemical reversible control of phase separation is visualized for the first time with alternating irradiation of UV and visible light.