The chemical durability of gamma-irradiated glass fibrous insulation commonly used in the reactor containment of nuclear power plants was tested by static leaching tests at 90°C. Distilled water and borate coolant solution were used as corrosive media. Two radiation doses, 2 and 4 MGy, were applied, the higher one roughly corresponding to 30 years of irradiation in reactor containment. The glass insulation was irradiated at low (70°C) and increased (450°C) temperatures. The results of the static leaching tests were compared with those obtained for nonirradiated native glass fibers. In distilled water, higher normalized leached amounts of calcium were found for low-temperature-irradiated glass fibers and in the initial stage of leaching of high-temperature-irradiated glass fibers; the lower normalized leached amounts were found for boron for glasses irradiated at both temperatures. In the borate coolant solution, higher normalized leached amounts of calcium and lower leached amounts of aluminum were observed for glasses irradiated at both temperatures. In all cases, the results were comparable for both applied radiation doses. Moreover, extraordinary brittleness of the glass fibers irradiated at high temperature was observed. This principally new finding needs further experimental and theoretical investigation.