Hydrothermal experiments have been performed investigating granodiorite-water, waste glass-water and granodiorite-waste glass-water systems in the temperature range 100–350°C at pressures of 50 or 60 MPa. Experimental equipment consisted of closed-system large-volume gas-pressurised reaction vessels. Run durations lasted from less than one day to one hundred days at fluid/solid mass ratios between 0.4 and 6. Fluids generated in the granodiorite-water experiments were alkaline (pH = 7.4–8.9) with low total dissolved solids (< 500 ppm) and low chloride (< 40 ppm) and sulphate (< 30 ppm) contents. Silica concentrations in solution approximate to theoretical values in the quartz-water system at 100° and 150°C, although 200°C fluids show evidence of re-equilibration during the quenching process. From available thermodynamic data, the chemistries of the 100° and 150°C fluids appear to be governed by feldspar-water equilibria, whilst at 200°C, feldspar-montmorillonite reactions dominate the fluids. Montmorillonite is an identified secondary phase at 200°C. From these preliminary data, the “evolved groundwaters” produced through reaction of granodiorite and water at high temperatures seem suitable for canister preservation, actinide immobilisation and compatibility with bentonite as a backfill material. Two sodium borosilicate glasses containing simulated waste components have been used in a kinetic study of dissolution at 100° and 150°C, 60 MPa. The dissolution of both glasses is governed by parabolic kinetics with run-lengths up to 14 days, and this is interpreted as being controlled by diffusion through a surface layer. This surface layer develops through incongruent dissolution of the glass and is enhanced by saturation of major components in solution and precipitation of secondary-mineral forms. The crystallinity of this alteration layer is increased by increasing time and temperature of the reaction. SEM and XRD analysis have indicated that at 100° and 150°C the layer is largely amorphous with traces of a poorly crystalline dioctahedral smectite. At 200°C the layer consists of fairly well crystallised smectite and at 350°C degradation of the glass is rapid and the solid reaction products are composed of a complex mineral assemblage including smectite, a lithium-sodium borosilicate hydrate, aegirine, riebeckite, albite, stillwellite, zektzerite and two barium molybdates. Some of these mineral phases incorporate and concentrate waste components, e.g., the zektzerite may contain up to 20 wt.% zirconium oxide, 2.5 wt.% caesium oxide and 0.7 wt.% uranium oxide. The rates of release of certain radionuclides have been investigated in granodiorite-waste glass-water systems. This study has revealed that the rate of release of a particular nuclide is a function of temperature and time. Caesium leach rates obtained in this manner are at least an order of magnitude lower than leach rates derived from refluxing dynamic leach tests. This is because incongruent dissolution of the glass, saturation of major components in solution and precipitation of mineral phases incorporating waste components in these closed-system experiments all serve to lower rates of radionuclide release. The work was carried out by N.E.R.C.-I.G.S. under contract to the U.K. Department of the Environment and the Commission of the European Communities.