The stabilization of thallium cuprates and oxycarbonates by molybdenum has been studied. Two cuprates have been stabilized: Tl 0 . 8 Mo 0 . 2 Sr 2 CuO 5 - δ with the @'1201@' structure which does not superconduct and the superconductor Tl 0 . 8 Mo 0 . 2 Sr 2 CaCu 2 O 7 - δ with the @'1212@' structure whose T c increases from 40 K, when as-synthesized, to 86 K after annealing in a reducing atmosphere. A new superconducting oxycarbonate Tl 0 . 8 Mo 0 . 2 Sr 4 Cu 2 (CO 3 )O 7 - δ has been synthesized, whose structure corresponds to an intergrowth of the @'1201@' structure of Tl 0 . 8 Mo 0 . 2 Sr 2 CuO 5 - δ with the Sr 2 CuO 2 CO 3 structure. This superconductor also exhibits a T c ranging from 30 K, when as-synthesized, to 80 K when annealed in a reducing atmosphere. Substitution of La for Sr in the @'1201@' phase allows superconductivity to be induced up to 40 K, whereas substitution of Ba for Sr in the oxycarbonate does not significantly influence the superconducting properties.