After the closure of a high-level waste repository, corrosion of the carbon steel overpack will occur. The corrosion products can then migrate into bentonite and affect the migration behavior of radionuclides in bentonite. Therefore, electrochemical experiments, with Fe2+ supplied by anodic corrosion of carbon steel, were carried out to study trivalent lanthanides in compacted bentonite. The interface between a carbon steel coupon and bentonite (dry density, 1.5 Mg/m3) was spiked with a tracer solution containing Nd(NO3)3, Eu(NO3)3, Dy(NO3)3, and Er(NO3)3. The carbon steel coupon was connected as the working electrode to a potentiostat and held at a constant potential between -550 and 0 mV (vs. Ag/AgCl reference electrode) for 7 days. A model using dispersion and electromigration could explain the measured profiles in the bentonite specimens. The best-fit electromigration velocity was related to the applied electric potential and was 1.0–3.8 nm/s for Nd, Eu, Dy, and Er ions. For these lanthanides, the best-fit dispersion coefficient was also related to the applied potential and was 0.8–1.6 μm2/s, and the dispersion length was calculated as 0.2 mm from the linear relationship between the dispersion coefficient and electromigration velocity. Finally, the apparent diffusion coefficient for these lanthanides was estimated as 0.6–0.9 μm2/s.