The complexation between (2-hydroxypropyl)-gamma-cyclodextrin (HP-gamma-CD) and water-soluble betulin derivatives, betulin 3,28-disulfate (DSB) and betulin 3-acetate-28-sulfate (ASB), belonging to the class of pentacyclic lupane triterpenoids, was studied using mobility shift ACE (ms ACE). It was found that the complexation is a high-affinity interaction. In this case, a very low amount of HP-gamma-CD should be added to the BGE, and triangular peaks are observed as a result of ligand deficiency in the sample zone. Le Saux et al. showed in 2005 that using the parameter a(1) of the Haarhoff-Van der Linde (HVL) function instead of the migration time measured at the peak apex eliminates the effect of ligand deficiency on effective electrophoretic mobility. Therefore, the electrophoretic mobilities of asymmetrical peaks of DSB and ASB were calculated in this way. The obtained experimental data correspond to 1:1 complexes. The calculated values of binding constants logarithms at 25 degrees C are 6.70 +/- 0.05 and 7.03 +/- 0.10 for the HP-gamma-CD complexes of DSB and ASB, respectively.