The crystal structures of potassium-exchanged forms of catapleiite and hilairite of the compositions (K0.49Ca0.42Na0.26) ZrSi3O9 · 2[(H2O)0.8(H3O)0.2] and K0.51ZrSi3O9 · [(H2O)0.5K0.27(H3O)0.23], respectively, were studied by X-ray diffraction and IR spectroscopy. Both structures retain the heteropolyhedral frameworks of the parent minerals formed by Zr octahedra and Si tetrahedra. The K cations occupy different positions in these minerals. In K-exchanged catapleiite, K cations are located only in the position occupied by Na in the structure of the parent mineral. In the K-exchanged form of hilairite, K cations are not only involved in the Na position but also partially occupy the H2O position.