Mixtures of poly(methyl methacrylate) (PMMA) and the random copolymer of the styrene and the acrylonitrile (PSAN) could be used to study the compatibility in monolayer films when the miscibility between these molecules was systematically varied by changing the content of acrylonitrile (AN) in PSAN. Surface pressure-area (pi-A) isotherms at the various compositions were measured on the water sub-phase and AFM images of the films transferred on Si-wafer were presented. Compatibility was shown to decrease as AN content was increased; 24.2 wt.% formed miscible blends with attractive interaction, 32.0 wt.% formed nearly ideal blend, and 52.0 wt.% manifested immiscible characteristics, respectively. These results were closely in accord with the known compatibility in the three-dimensional state.