The simultaneous diffusion of two isotopes of a given ionic impurity in an electric field is considered, and an improved method of analyzing the experimental data to obtain &Dgr;D/Dis discussed. (HereDis the diffusion coefficient of one of the isotopes, and &Dgr;Dthe difference between those of the two isotopes.) As noted by Chemla, an electric field increases the accuracy with which &Dgr;D/Dcan be determined. Equations are found giving this increase in accuracy as a function of field and diffusion time. If the ratiow2&agr;/w2&bgr;of the jump frequencies of the two isotopes is known, relative values of the vacancy or interstitialcy jump frequencies near an impurity can be calculated from &Dgr;D/Dand &mgr;/D, where &mgr; is the drift mobility. As an example, data in the literature for Na22and Na24diffusing in KCl are considered. For self‐diffusion,w2&agr;/w2&bgr;may be calculated from &Dgr;D/Dand known values of the correlation factor. When data in the literature are used to estimatew2&agr;/w2&bgr;for Na22and Na24diffusing in NaCl, a value smaller than that predicted by classical rate theory is found.