Abstract Leach tests were conducted on 0.5 g disc samples of SYNROC and two glass types using distilled water at 85 and 200°C. No leaching was detected for SYNROC at either temperature. Thus, the upper limit on leach rate for SYNROC is < 0.005 g/m 2 d. Waste glass PNL 76-68 had leach rates of 1.4 g/m 2 d at 85 °C and 8.9 g/m 2 d at 200 °C, while 73-1 glass frit had a leach rate of 41 g/ m 2 d at 200 °C. The leach tests were repeated in the presence of rock powders. Again, no leaching was measurable for SYNROC. PNL 76-68 glass had leach rates between 4 and 23 g/m 2 d at 200 °C and 73-1 frit leached at rates between 29 and 176 g/m 2 d at 200 °C. Tests were also conducted on crushed glass samples (PNL 76-68, 100–200 μm size fraction). Bulk leach rates were calculated based on measurement of Ca, Cs, and U in the leach solutions. Leach rates for Cs at 95 °C were in good agreement with the results on bulk sample weight loss at 85 °C. Ca leach rates were slightly lower. At 200 °C, Cs leach rates were similar to bulk sample weight loss while U leach rates were a factor of 5 to 9 lower. The results of the leach tests show that SYNROC is several orders of magnitude more resistant to leaching than glass.