Isotachophoretic qualitative indices, RE, of twenty-six amino acids were measured at several pHL values in the range of 8.6-9.6. The absolute mobility, m0, and pKa values were evaluated by the use of a least-squares method utilizing a simulation of the isotachophoretic steady state. The pKa values were in good agreement with values cited in the literature. The RE values simulated using the evaluated constants were in good agreement with RE values converted from step heights observed previously. By comparing the previously observed separation behaviour of amino acids with their simulated effective mobilities, it is concluded that when the effective mobility of samples differs by ca. 1 X 10(-5) cm2 V-1 s-1 at the steady state, ca. 10-nmol samples can be separated by the use of a 80 cm X 0.5 mm I.D. tube. The simulated effective mobilities of twenty-two amino acids were tabulated to assess the separability under some typical electrolyte conditions.