Clay minerals such as montmorillonites and vermiculites, and zeolites that are candidate overpack materials, such as clinoptilolite, chabazite, phillipsite, mordenite, and erionite were treated under hydrothermal conditions of 200 and 300°C with a pressure of 30 MPa for 28 days. X-ray diffraction (XRD) analysis revealed that montmorillonites did not seem to alter while vermiculites were partially chloritized by hydrothermal treatment at 200 and 300°C. Natural zeolites, excepting phillipsite, did not seem to have transformed at 200°C but transformed to various extents at 300°C. Selective sorption of cesium and strontium decreased to different degrees in all the above minerals excepting two montmorillonites and mordenites after hydrothermal treat ment because of the nature and extent of mineral transformation under these hydrothermal conditions as revealed by XRD and cesium and strontium sorption measurements. For examples, cesium sorption Kd (ml/g) values decreased from 8100 to 1000 and 4600 to 2800 in erionite and chabazite, respectively, while they decreased drastically from 2600 to 90 and 3900 to 70 in vermiculite and phillipsite, respectively, after hydrothermal treatment at 300°C. Montmorillonites among clay minerals and mordenites among the zeolites studied here seem to be very resistant to hydrothermal alteration and therefore may be preferred in an overpack. These results point out that the effects of heat on mineral transformations should be considered before the selection of proper overpack materials.