A copolymer of acrylonitrile and methyl acrylate (ANMA(11)) was synthesized and subjected to monolayer studies. The monolayer studies were done at the air-water interface. Water and aqueous solutions of sodium dodecyl sulfate (SDS) were used as the subphase and the compression isotherms were noted. Different concentrations of SDS in the subphase, leading to different surface pressures, affected the pi - A isotherms. Free energy of compression was computed from the pi - A isotherms for different systems. The values were also computed for homopolymers:poly(methyl acrylate)(PMA), polyacrylonitrile (PAN), and their physical mixture. The results are analyzed and discussed.