An understanding of the behaviour of contaminants in compacted clays is important in assessing the effectiveness of clay-based barrier materials used in many waste containment strategies. Here the diffusion and sorption behaviour of I− with selected compacted clays is examined (129I is a relatively long-lived radioisotope present in high-level nuclear fuel waste.) Diffusion coefficients, D, and distribution coefficients, Kd , were measured for I− with four clays: bentonite, Lake Agassiz clay (a glacial lake clay composed mainly of smectite, illite, kaolinite, and quartz), interstratified illite-smectite, and kaolinite. For the diffusion experiments the clays were compacted to a dry bulk density, ρ, of ∼1.2 Mg m−3. The mean measured D values, D m, were as follows: bentonite, 310 µm2 s−1; Lake Agassiz clay, 0.62 µm2 s−1; illitesmectite, 190µm2s−1; and kaolinite, 74µm2s−1. The measured values were compared with those calculated, D c, from the following model: D=D oτa n/(n + ρKd ), where Do is the diffusion coefficient in pure bulk water, τ a the apparent tortuosity factor, and n the solution-filled porosity of the clay. Except for the Lake Agassiz clay, where D m was about an order of magnitude lower than D c, the D m and D c values agreed within a factor of about two. The Lake Agassiz clay has a markedly higher organic carbon content than the other three clays, and this could affect τ a, which may be overestimated in the model.