The operation of preparative isotachophoresis with either or both the leading and trailing ions replaced by the ions with the highest and lowest mobility in the feed is proposed. The advantages of this procedure are that one does not have to search for appropriate leading and trailing ions, and no additional chemicals need to be added and later recovered from the separation. The familiar steady-state theory for isotachophoresis predicts that the separation will be successful if the bands form. Experiments with the analogous displacement chromatography system were successful.