A wide variety of plasma treatments was performed on polyethylene surfaces, resulting in a wide range of total surface energies. The linear correlation of [GRAPHICS] with cos theta was discussed in light of the Young-Dupre equation. Hundred percent of the surface energy variation was accounted for by the polar component of surface energy; the dispersive component was not affected by surface treatment. These data show that for this polymer the contact angle of a single polar liquid can be used as a robust quantitative indicator of treatment level, and because of its excellent linear correlation with total surface energy for this system, can be used as a quantitative measure of total surface energy.