Capillary electrophoresis (CE) has been used to characterize humic acid (HA) aggregation. It was found that when pumping HA solution through the capillary at a constant flow rate with no electric field, the number of spikes could be reduced by filtration of the solution. Applying high voltage (30 kV), the amount of spikes increased again. This is associated with the formation of new aggregates caused by electric field. Aggregation is influenced by the concentration of HA, applied voltage and presence of imidazole in the solution. It is supposed that this phenomenon is characteristic of all solutions that can form colloidal particles: in beta-cyclodextrin solution, similar spikes appear in a high-voltage electric field.