[en] The release of polyvalent radionuclides from a nuclear waste repository located in basalt may be sensitively related to the redox potential (Eh) imposed by the basalt. A series of tests are reported here, evaluating the effect of basalt on the concentrations of ⁹⁹Tc released into solution from a borosilicate glass waste form. Crushed PNL 76 to 68 glass, doped with 0.7 mg ⁹⁹Tc/g glass, was reacted with reference basalt groundwater under oxic hydrothermal conditions in a sampling autoclave, both alone and in the presence of crushed basalt. The steady-state fluid concentrations of ⁹⁹Tc and various stable species were determined from samples obtained at the test conditions of 200⁰C, 30 MPa, and an initial solution-to-solid mass ratio of 10 for both tests. In the glass + groundwater test, the ⁹⁹Tc concentration rose rapidly to about 50 mg/L after only 200 hr of run time and remained at a value between 50 and 60 mg/L throughout the duration of the test. For the basalt + glass + groundwater test, the ⁹⁹Tc concentration rose to an initial value of about 2.5 mg/L. At about 700 hr, the ⁹⁹Tc concentration began to drop rapidly until a value near the analytical detection limit (approximately 0.01 mg/L) was reached after a test duration of 1400 hr. It is concluded that the presence of basalt in these hydrothermal experiments reduces the concentration of ⁹⁹Tc in solution by nearly four orders of magnitude, probably by control of solution Eh and subsequent precipitation of a solid containing a reduced form of technetium. Reaction mechanisms are discussed that can account for these observations. 17 references, 1 figure