Abstract: Ganophyllite has been shown to have alkali cation exchange capability. Partial cesium exchange for K + Na in large (> 0·3 mm) grains shows that the exchangeable cations migrate parallel to X, the crystallographic direction for the ‘interlayer’ tunnels. Such exchange capability supports the suggestion that the alkali elements are located in zeolite-like sites attached to the sides of the tunnels. Exchange experiments show that complete sodium substitution for potassium is possible also. Eggletonite, the Na analogue of ganophyllite, is shown to have an identical superlattice to ganophyllite, indicating that eggletonite differs from ganophyllite only by the exchangeable cation.