The apparent diffusivities (Da) and activation enthalpies (ΔEa) for I- and Cs+ ions in compacted Na-smectite were obtained in the parallel and perpendicular directions to the orientation of smectite particles as a function of smectite's dry density (0.9-1.4 Mg/m3), salinity ([NaCl]=0.01, 0.51 M) and temperature (295-333 K). The Da-values for both ions tended to be higher in the parallel direction than in the perpendicular direction to the orientation of smectite particles. The Da-values off I- ions in the parallel direction decreased with increasing salinity only at low dry density, but those of Cs+ ions increased with increasing salinity for all conditions. Based on this, it is interpreted that I- ions predominantly diffuse in external pores and Cs+ ions diffuse in both interlayer and external pores. The ΔEa-values for I ions are at similar levels as those for the ionic diffusivity in free water (D o) of I- ions at low dry density and increased with increasing dry density. The ΔEa-values for Cs+ ions are higher than those for the Do of Cs+ ions even at low dry density and increased with increasing dry density. Such high ΔE a-values for Cs+ ions are considered to be due to the combined effects of the ion exchange enthalpy between Cs+ and Na + ions in smectite and the lowering in the activity of porewater.