The formation of pure and mixed monolayers of 4-n-octyl-4’-cyano-biphenyl (8CB) and stearic acid at the air-water interface is investigated by a combination of surface balance measurements and polarization sensitive, optical second harmonic spectroscopy. These measurements allow one to identify four different regimes upon compression of the monolayer. These regimes are an isotropic monolayer, a polar oriented phase, a compacting of the polar oriented phase, and the formation of a centrosymmetric structure. From a fit of the polarization dependence of the second harmonic intensity, the orientation of the optically nonlinear 8CB chromophores is deduced. A comparison of monolayers of different mixing ratios of 8CB and stearic acid shows, that the orientation of the 8CB molecules depends only on the surface pressure and not on their concentration in the film.