[en] The solubilities of technetium (IV) oxides have been determined in solutions of NaCl, HCl, and synthetic groundwaters in the pH range 0 to 10. Oxides were prepared either by electroreduction from Tc(VII) solutions onto platinum electrodes or by precipitation of Tc from Tc(VII) solutions using hydrazine. In some of the determinations, the oxide was precipitated onto purified sand by use of hydrazine. The oxide-covered sand was then placed into a column and the test solution continuously recirculated through the column. The oxide plated on platinum was placed into a small cell and the test solution stirred. Solubilities were determined by measuring the beta radiation of ⁹⁹Tc in the stirred or circulated solution in contact with the oxide. In the pH range 4 to 10, solubilities were in the range 7.5 x 10^-9 to 5 x 10^-8 mol/L, and most of the determinations were in the range 1 x 10^-8 to 2 x 10^-8 mol/L. In acid solutions the solubilities were generally higher for hydrazine-precipitated oxide than for the electrodeposited oxide, and they increased as the pH was lowered. Solubilities of Tc(IV) oxides will be useful for calculation of transport rates of technetium for the case of solubility-limited transport in nuclear waste repository environments