Superconductivity with a maximum T c, zero of 58 K for x > 0·75 has been observed in a nominal starting composition, Th x Tl2Sr2Ca2Cu3O y . X-ray data show that the 1212 phase gets stabilized due to part substitution of Th at the Tl-site for x≥0·25; however, T c is low for x<0·75. © 1991 Indian Academy of Sciences.