Superconductivity with a Tc,zero at 75 ± 1 K has been observed in TlMBaCuO (M = Ce, Th) system for a starting nominal composition of Tl2.2M1Ba2Cu3Oy. X-ray data show that the compounds are multiphasic with the 2201 phase as the predominant one which is responsible for superconductivity. Lower Tc values are noted for M = Pr and Tb. Only metallic behavior is observed for pure Tl2Ba2Cu1Oy and M = Pb or (Pb + Sb) or Te containing phases down to 15 K. © 1990.