Isotachophoresis has the unique feature that both qualitative and quantitative information can be deduced from step heights in isotachopherograms, even for unknown ionic species. For strong cations and anions there exists a linear relationship between, respectively, the measured step heights and the response factor (RF) versus the specific zone resistance at 25°C (SZR25) for a specific leading electrolyte. Applying two standard components, e.g., the leading ion and Na+, the relationship between measured step heights and the SZR25 values can be set up and from the step heights of the sample ionic species in the isotachopherograms the SZR25 values can be obtained. With these values all other parameters such as the mobilities, the concentrations in the zones and the RF values can be calculated. In combination with the measured zone lengths, the molar fractions and mass fractions can be calculated. In this way, the number-average and weight-average molecular masses can be obtained for homologous series of anions/cations from the isotachopherograms of the mixtures. This procedure is applied to jeffamines D230 and D400 and the obtained molecular masses are comparable with the labelled values.