A detailed investigation has been made of the comparative abilities of SYNROC and borosilicate glasses to immobilize high level nuclear wastes for extremely long periods of time when buried in geological repositories. Under dynamic leaching conditions (glasses tend to dissolve quasicongruently whereas the dissolution of SYNROC minerals is highly incongruent. Loss of low valence cations (e.g., Ca, Ba, Cs) from the near-surface regions of SYNROC titanate minerals results in surface films consisting dominantly of titanium dioxide. Because of the extreme insolubility of TiO 2 in water, these films tend to protect the surface from further dissolution and hence the leach rates of SYNROC minerals fall off drastically with time, commonly by 2 orders-of-magnitude after 10–30 days of dynamic (undersaturated) leaching. In sharp contrast, the leachability of a typical borosilicate glass (PNL 76-68) remains constant with time under the same conditions. Because of the incongruent leaching mechanisms, the leachability of different elements from SYNROC can vary by factors of several hundred. Thus, it is impossible to describe the leachability of SYNROC by any single, characteristic value at a given temperature. Detailed studies have been made of the leaching behaviour of borosilicate glass, SYNROC and SYNROC minerals under quasidynamic conditions in water at 95 and 200°C. After relatively short periods (10–30 days) at 95°C the leachabilities of cesium, calcium and uranium from SYNROC are smaller than those of borosilicate glass by factors of 500, 2500, and 100,000, respectively. Strontium and barium have similar leach rates to Ca, and the leachability of rare earths and trivalent actinides is expected to be intermediate between those of Ca and U. Less extensive leaching data obtained at 200°C show that the differences in leachability between glass and SYNROC are greater than at 95°C. Results of leaching studies of individual SYNROC minerals are consistent with measurements on bulk SYNROC samples. Taken together, the results demonstrate that SYNROC is more resistant to leaching than borosilicate glass. The effects of radiation damage on SYNROC minerals have also been investigated. The amount of alpha particle irradiation which would be received during the decay of radwaste in SYNROC minerals to safe levels causes only minor structural changes in the zirconolite and perovskite minerals which are the hosts for a-emitting actinide elements. Tracer measurements show that these minerals behaved as closed systems for U, Th, and Pb daughter products despite suffering radiation doses more than 10 times as intense as would be achieved during decay of contained radwaste to safe levels. Experimental studies demonstrate low leachability of these minerals despite their exposure to large radiation doses. The superior leach resistance of SYNROC at elevated temperatures will make it possible to bury it in geologic repositories within a few years after reprocessing, thereby avoiding the necessity for prolonged storage and supervision. It should be possible to immobilize about three times the weight of HLW calcine in a given volume of SYNROC than is now envisaged for glass.