The process of humic acids (HAs) component agglomeration (oligomerization) can be followed by capillary zone electrophoresis (CZE). Oligomerization of HAs of different origin was found to be strong, it leads to kinetically robust entities which can be separated by CZE and their formation is one of the reasons for unsuccessful separation by previous authors. Another difficulty comes from the sorption of some of the HA fractions on the fused-silica capillary wall. Critical concentration for the formation of aggregates was found to be mainly around 30 mg/l. Results of CZE were confirmed by vapour pressure osmometry.