Activation of polymeric surfaces, i.e. formation and/or modification of the functional groups on the surface of a material, is essential prior to the further processing of polymers, especially in applications where wettability plays a crucial role. In this study, an atmospheric pressure ambient air plasma treatment of poly(styrene) (PS) and poly(ethersulfone) (PES) foils using diffuse coplanar surface barrier discharge is presented. The plasma treatment for 0.5 s resulted in a decrease of water contact angle from the original value of 83 degrees to 26 degrees for PS and from 76 degrees to 32 degrees for PES. No significant changes in wettability were observed for prolonged treatment times. Better wettability was correlated with decreasing carbon to oxygen ratio resulting from an incorporation of oxygen-containing functional groups C-OH, C=O and O-C=O on the surface. X-ray photoelectron spectroscopy was employed to study details in the changes of the surface chemistry following the plasma exposure. We used atomic force microscopy to study the formation of low molecular weight oxidized material (LMWOM) during the plasma treatment. After dissolving the LMWOM in water, we observed roughening of the plasma-treated surfaces at the nanometre level due to etching induced by plasma treatment.