Surface activity of N-monopropyl maleamic acid-alt-styrene (NPMA-alt-S) and N-monodecyl maleamic acid-alt-styrene (NDMA-alt-S) was studied. Surface pressure-area isotherms (Pi-A) at the air/water interface were determined. It was found that the pH of the water subphase and the chemical structure of the copolymers studied strongly influence the type of isotherms. It was also found that the driving force for the adsorption of NPMA-alt-S, water soluble polymer at the air/water interface arises from an entropic contribution. (C) 2004 Elsevier Ltd. All rights reserved.