Silver tellurite (Ag 2TeO 3) was prepared from an aqueous solution and also by oxidation of Ag 2Te in air at 973 K. Pure, stoichiometric Ag 2TeO 3 was obtained by recrystallization from an ammoniacal solution. The phase crystallized from aqueous media (α) is orthorhombic, while the high-temperature preparation is monoclinic (β). The α-phase is metastable and transforms irreversibly to the β-phase above 573 K. DTA studies showed that Ag 2TeO 3 melts congruently at 857 K. The phase diagram in the composition region of Ag 2TeO 3TeO 2 was determined. It showed a eutectic at 710 K and 70 mole% TeO 2 of the Ag 2OTeO 2 system. The isothermal section at 700 K of the ternary phase diagram of AgTeO system was constructed from the results of this study. The free energy of formation of Ag 2TeO 3 is >-65 kcal/mole. This upper limit of stability indicates that it cannot form in a nuclear reactor. Freezing behavior of Ag 2OTeO 2 melt showed a tendency to glass formation in the composition range of 61-72 mole% TeO 2. Freezing of the melt also appears to produce "tellurite," the naturally occurring mineral form of TeO 2.