Films of high-density polyethylene (HDPE) have been sulfonated by solid-gas phase reaction using gaseous sulfur trioxide (SO3) of different concentrations in nitrogen. The effect of concentration of SO3 in an SO3 + N-2 gas mixture and time of sulfonation on solvent and gas barrier properties of sulfonated HDPE films was studied by determining toluene and oxygen permeability of HDPE films sulfonated with different concentrations of SO3 for different times. The color developed in sulfonated HDPE films during sulfonation could be bleached by aqueous sodium hypochloride solution. The effect of sodium hypochloride solution treatment on oxygen permeability of sulfonated HDPE films was also studied.