Abstract: The effect of orientation of vermiculite particles on the diffusion of Na ion was measured by a tracer technique. The diffusion measurements were made on pellets prepared by pressing freeze-dried vermiculite in a cylindrical die and the diffusion coefficient evaluated by a thin-film boundary condition of Fick's law. Because flakes of vermiculite were highly oriented under the pressing force, it was possible to prepare different angles of specimen orientation with respect to the surface where diffusion was initiated. Mathematical relationships of orientation angle, axial ratio of the platelets, and apparent diffusion coefficient were developed. The experimental results on the diffusion of Na ion in K-vermiculite of clay size gave apparent diffusion coefficients of 2·21 × 10−7 cm2 sec−1 when the diffusion flux was parallel with the clay platelets and 0·45 × 10−7 cm2 sec−1 when the flux was perpendicular to the clay platelets. For the diffusion of Na ion into Na-vermiculite, apparent diffusion coefficients were obtained of 1·18 × 10−8 cm2 sec−1 when the diffusion flux was parallel with the clay platelets, and 0·18 × 10−8 cm2 sec−1 when the flux was perpendicular to the platelets. For K-vermiculite, the change in apparent diffusion coefficient is a simple one of particle geometry. The Na-vermiculite system exhibited more complicated behavior.