Because of the mixed metal valance state of thallium in a Tl‐based superconductor, a nearly single ‘‘1223’’ phase can be readily prepared by a modified Pechini method (US Patent 3 330 697). Tl2O3and other metal nitrates are dissolved in an aqueous solution plus a certain amount of oxalic acid for chelating the metal cations. The desired phase shows zero resistance at 114 K and the phase purity is identified by x‐ray diffraction. Characteristics of superconductivity also are evidenced by the Meissner effect. The preparation technique is considered valuable to purify multiphase superconductors for mass production.