The miscibility in binary polymer systems in 2-dimensional state at the air/water interface was investigated by the relationship between the mean molecular area at a constant surface pressure and the mole fraction of one component in the binary mixtures of poly(methyl methacrylate)(PMMA) and poly(vinyl acetate)(PVAc). Compatibilization of the immiscible PMMA/PVAc blend at the air/water interface was achieved by introducing carboxyl acid to either of the two component polymers.