Haeffner (1953) discovered that the isotopes of liquid mercury could be separated by an electric field, with the lighter isotopes being preferentially enriched near the anode. The authors consider the analogous problem for a gaseous, fully-ionized, binary isotopic plasma and it is demonstrated that isotopic separation occurs in the same way, i.e. the lighter isotopes are preferentially located near the anode. A previous analysis by Frie (1963) is discussed.