Diffusion coefficients are critical parameters for predicting migration rates and fluxes of contaminants through clay-based barrier materials used in many waste containment strategies. Cabon-14 is present in high-level nuclear fuel waste and also in many low-level wastes such as those generated from some medical research activities. Diffusion coefficients were measured for C-14 (in the form of carbonate) in bentonite compacted to a series of dry bulk densities, rho(b), ranging from about 0.9 to 1.6 Mg/m3. The clay was saturated with a Na-Ca-Cl-dominated groundwater solution typical of those found deep in plutonic rock on the Canadian Shield. Both effective, D(e), and apparent, D(a), diffusion coefficients were determined. D(e) is defined as D0tau(a)n(e), where D0 is the diffusion coefficient in pure bulk water, tau(a) the apparent tortuosity factor, and n(e) the effective porosity available for diffusion; and D(a) is defined as D0tau(a)n(e)/(n(e) + rho(b)K(d)), where K(d) is the solid/liquid distribution coefficient. Both D(e) and D(a) decrease with increasing rho(b): D(e) values range from about 10 x 10(-12) m2/s at rho(b) congruent-to 0.9 Mg/m3 to 0.6 X 10(-12) M2/s at 1.6 Mg/m3, and D(a) values vary from approximately 40 X 10(-12) to 4 x 10(-12) M2/s over the same density range. The decrease in D(e) and D(a) is attributed to a decrease in both tau(a) and n(e) as rho(b) increases. The data indicate that n(e) is < 10% of the total solution-filled porosity of the clay at all densities. K(d) values for C-14 with the clay range from about 0.3 to <0.1 m3/Mg; this indicates there is a small amount of C-14 sorbed on the clay and/or some C-14 is isotopically exchanged with C-12 in carbonate phases present in the clay. Finally, the D(e) values for C-14 are lower than those of other diffusants - I-, C1 -, TcO4-, and Cs+ - that have been measured in this clay and pore-water solution. This is attributed to lower values for both n(e) and D0 for C-14 species relative to those of the other diffusants.