A special vacuum process has been developed for adding an OH functional group onto polymer surfaces with different reactivities. In this novel technique, polymer surfaces were exposed, in a vacuum chamber, to OH radicals generated by the dissociation of water vapor on hot rhenium wires. We found by x-ray photoelectron spectroscopy and high resolution electron energy loss spectroscopy that about half a monolayer equivalent of C-OH groups could be generated on a relatively reactive polymer like polystyrene with a 10 s exposure at a water pressure of 1x10(-3) Torr. However, under the same experimental conditions, the oxygen incorporation on more stable polymers was much lower; it was reduced by 90% on polyethylene and to practically zero on Teflon. For surface engineering of these stable polymers, thermionic electrons emitted from the hot filaments were used to facilitate electron impact ionization of some of the water vapor. Positive ions from the ionization region were then extracted and accelerated by an electrostatic field toward the polymer target which was separated from the ionization region by an adjustable distance about twice the gas-phase collision mean-free path. Although ion bombardment of the polymer surface was limited by the insulating property of the polymer, ion-molecule collisions in the gas phase were efficient with such a setup in generating energetic neutrals flying toward the polymer surface for the activation of the polymer. With an accelerating voltage of 50 V and a water pressure of 1 x 10(-3) Torr (the collision mean-free path being about 5 cm), about a 0.2 monolayer equivalent of COH was added to Teflon and polyethylene in 60 s.