Abstract Wet-chemical analysis of the thallium and oxygen content of the 2223 phase Tl 2− x Ba 2 Ca 2 Cu 3 O 10− x has been used to establish its stabilization over the compositional range 0.35 y y ≤0.3. Samples reacted in sealed silica tubes always gave the 2212 phase Tl 2− y Ba 2 CaCu 2 O 8− x if fired at or below 860°C or if the final product contained an atomic ratio Tl/Ca > 0.83. Nucleation occurs only above 860°C if sealed at 1 atm air; it may be reduced to 830°C if samples are sealed at 10 −5 Torr. Transformation of a sample from the 2212 to the 2223 phase is completed within 2 h at 880°C in the presence of a ratio Tl/Ca y and x mutually adjust so as to form a superconductive phase with T c ≈116 to 118 K under normal firing conditions, viz. 880°C for 2 h followed by 860°C for 2 h. Annealing above 700°C leads to a further loss of thallium and oxygen, but the chemical composition remains unaltered even for long times at 700°C. A 120 h anneal at 700°C increased T c from 117 to 126 K as well as the Meissner fraction of a 2223 sample that was single-phase according to X-ray diffraction. The improvement in superconductive properties is deduced to result from a homogenization of the Tl + ions in the tl 2− y O 2− x layers.