[en] Diffusivities were measured for plutonium in brine-saturated compacted Wyoming bentonite. Complexities of the solution chemistry and retardation of transuranics necessitate diffusion studies under conditions that are specific for repository host rock types, in this case salt. Diffusivity values in the range of 10^-15 to 10^-14 m²/s were obtained for bentonite at a packing density of 1800 kg/m³. That density was obtained by compaction at 15 MPa, a typical lithostatic pressure in a repository in salt at 650 m depth. Even a 0.05-m (2 inch) thick bentonite-containing engineered barrier could decrease radionuclide release rates by approximately 4 orders-of-magnitude if the diffusivity for that radionuclide were in the observed range of 10^-15 to 10^-14 m²/s. These results confirm the effectiveness of uncompacted bentonite-containing materials as engineered barriers for radioactive waste isolation. 23 references, 2 figures, 1 table