Variable concentrations of low molecular weight poly(methylphenylsiloxane) were added to polyimide at its precursor poly(amic acid) stage to produce blends of different compositions. Ultimately cured poly(methylphenylsiloxane)/polyimide blends (PMSI) were exposed to radio frequency oxygen plasma. The stability of the blends to the oxygen plasma was assessed by monitoring the weight loss of the films as a function of exposure time. After 24 h exposure blends exhibited weight losses that were much lower than neat polyimide. The effect of oxygen plasma on polyimide and its blends has been analyzed by scanning electron microscopy and changes in surface chemistry of the exposed films were subsequently examined using X-ray photoelectron spectroscopy. The mechanical properties of oxygen plasma exposed and unexposed neat polyimide and its blends have been evaluated. (C) 2004 Kluwer Academic Publishers.