X-ray photoelectron spectroscopy and near edge X-ray absorption fine structure investigation have been used to characterize the surfaces of two oxygen plasma-treated high-density polyethylene (HDPE) samples having different crystalline fractions. Both the observations indicate that a higher degree of crystallinity restricts the mobility of the polar functional groups on the HDPE surface. An increased crystalline order lowers the amount of oxidation and the aging of polar functional groups on the substrate surface. The results are supported by contact angle measurements and field emission scanning electron microscopic observations.