The fundamental equation of isotachochromatography, i.e., isotachophoresis translated into ion-exchange chromatography, has been derived for weak acids and weak bases. Weak acids are separated on strong cation exchangers and weak bases on strong anion exchangers. According to theory, the elution profile (a plot of pH versus elution volume) should exhibit a plateau for each component. The length of a zone is proportional to the amount of the compound concerned. In the absence of adsorption of the compounds to the matrix, the pH levels of succeeding plateaux will differ by values equal to the difference in the pKa values of the succeeding compounds. The term "isotachochromatography" is discussed.