Abstract A cubic ( a=18.2886 A ̊ ) thallium-doped barium cuprate of composition Ba 1-xTl xCuO 2+y, where x=0.1−0.15 and y=0.12, in which Ba(II) sites are doped by Tl(I,III), has been synthesized. A full x-ray structure refinement has been performed on a single crystal isolated from a ceramic preparation. Typical ceramic preparations show metallic behaviour and zero resistance between 25 and 35K. However, since Meissner fractions range from 1 to 3%, the superconductivity is probably associated with a grain boundary region where either intergrowth of the layered Tl 2Ba 2CuO 6 phase or optimal Tl and concomitant oxygen doping occurs. K-doping of the barium cuprate phase and Tl-doping of a related Ba 2Cu 3O 5 phase are also briefly discussed.