The behavior of mixed monolayers of cholesterol and poly(methyl methacrylate) (PMMA) with molecular weights of M-w = 120 000 g/mol and M-w = 5 000 g/mol was investigated at the air/water interface using Langmuir and Brewster angle microscopy techniques. From the data of surface pressure (pi)-area (A) isotherms, compressional modulus-surface pressure (C-s(-1)-pi) curves, and film thickness, complemented with Brewster angle microscopy images, the interaction between the components was analyzed. Regardless of the surface pressure (pi = 10, 20, or 30 MN/m) at which the mean molecular/monomer areas (Am) were calculated, the Am-mole fraction plots (corresponding to X-PMMA = 0.1, 0.3, 0.5, 0.7, and 0.9) show that all the experimental points obtained are placed on the theoretical straight line calculated according to the additivity rule. This fact, together with the existence of two collapses in the mixed monolayers and with the fact that the surface pressure of the liquid-expanded LE-L'E phase transition of PMMA does not change with the monolayer composition, demonstrates the immiscibility of the film components at the interface. The application of the Crisp phase rule to the phase diagram of PMMA-cholesterol mixed monolayers helps to explain the existence of a biphasic system, regardless of their composition and surface pressure. Besides, Brewster angle microscopy (BAM) images showed the existence of heterogeneous cholesterol domains with high reflectivity immersed in a homogeneous polymer separate phase with low reflectivity.