Three independent speciation techniques, the free liquid/moving boundary electrophoresis, ultrafiltration, and equilibrium dialysis, combined with the radiotracer method (using 152Eu) have been compared at the study of Eu interaction with humic acid (HA). The degree of complexation of Eu in 10 mg/l Aldrich HA solutions was determined within a broad range of metal loading (Eu total concentration 10-8-10-4 mol . l-1), at pH 4 and 6, ionic strength of 0.01 and 0.1 (NaClO4). From the anodic electrophoretic mobilities determined, additional information on the charge of the Eu-HA complexes was obtained. Uncertainty of the determination given by sorption losses of Eu on the walls and membranes of the experimental devices, and the effect of kinetic lability of the Eu-HA complexes are estimated. Based on the ultrafiltration experiments, the existence of an unknown, labile, high-molecular non-humic Eu species is discussed.