Through-diffusion experiments in bentonite with humic colloids in the size range of 1-10 nm were carried out. Bentonite was compacted to 0.6-1.8 g/cm 3 dry density and equilibrated with 0.01 and 0.1 M NaClO 4 solutions. Experiments with Eu(III) diffusing in the absence and presence of humics were run in parallel, as well as Co(II) diffusing through compacted bentonite in the presence of humics. The humic colloid diffusion experiments were run for 60 days and the humic concentration in the outlet solutions measured at time intervals. The experimental breakthrough curves for humic substances (HS) as well as the HS, Co(II) and Eu(III) profiles in the bentonite were simulated using the finite difference based computer code ANADIFF. Regardless of the compaction and ionic strength of solutions, humic colloids diffused through the compacted bentonite. The effects of humic colloids on both Co(II) and Eu(III) sorption as well as on diffusion were significant. The apparent diffusivity ( Da) increased significantly for both Co(II) and Eu(III) when humic colloids were present and the distribution coefficient ( Kd) values decreased.