TlBa2Ca2Cu3Ox (Tl-1223) powder was synthesized in O2 from a mixture of Tl2O3 and Ba2Ca2Cu3Ox. The peritectic melting point of TI-1223 in O2 is 905-degrees-C. It was found that the maximum phase conversion to the Tl-1223 phase occurred at 895-900-degrees-C. A small concentration of liquid was present at that temperature. Because of the rapid kinetics of Tl-1223 formation, CaO and BaCuO2 phases became isolated from the Tl2O3 and thus remained in the final microstructure. In samples heated at 895-degrees-C, the CaO and BaCuO2 proved to be sufficiently small to act as flux-pinning sites.