In order to functionalize the surface of blown low-density polyethylene (LDPE) and cast polypropylene (CPP) films, and ultimately to maximize the attachment of active molecules onto them, the optimum treatment parameters of capacitively-coupled radio-frequency (13.56 MHz) oxygen plasma were investigated by using contact angle, toluidine blue dye assay, X-ray Photoelectron Spectroscopy (XPS) and Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR). Contact angle values of LDPE and CPP samples decreased significantly after oxygen plasma treatment. They further decreased as the plasma power level increased. The treatment time had no substantial effect on contact angle value. The optimum treatment conditions for LDPE and CPP films for maximizing carboxyl functionality without causing observable surface changes were found to be 200 W/200 mTorr and 250 W/50 mTorr, respectively, when treated for 3 min. The maximum carboxyl group concentration obtained with LDPE and CPP films were 0.46 and 0.56 nmol/cm2, respectively. The percent of oxygen atoms on the surface of plasma-treated LDPE and CPP films was determined by XPS analysis to be 22.6 and 28.7%, respectively. The ATR-FTIR absorption bands at 1725–1700 cm−1 confirmed the presence of carboxylic acids on LDPE and CPP films. By exposing the plasma-treated sample to air rather than water and treating films repeatedly with oxygen plasma, a higher carboxyl group concentration could be obtained. Copyright © 2008 John Wiley & Sons, Ltd.