Abstract: Bulk thallium based superconductors with nominal starting composition of (Tl2-xMbxBa2Ca2Cu3Ox ) were prepared through a solid-solid reaction technique. A Ba-Ca-Cu-O precursor was made for this purpose by mixing and grinding the stoichiometric amounts of BaCO3, CaCO3 and CuO followed by sintering at 850oC. Finally, appropriate amounts of MoO2 and Tl2O3 were added to the precursor and the whole mixture was calcined at 950oC under oxygen flow. Different phases including the high- Tl-2223, Tl-2212, Tl-1223, Tl-1212 phases were formed in the resulting compound. Some other phases were also found when the Tl to Mo ratio was changed from 1.6 : 0.4 to 1 : 1 in Tl1.6Mo0.4Ba2Ca2Cu3Ox superconductor. From the resistivity measurements, the critical temperature Tc(0) for the compound was found to around 89 K. The doping of Molibdenum (Mo) in Tl-cuprates induces pairing mechanisms to occur outside the CuO2 layers and thus reducing high-Tc phase formation and hence the Tc(0) value.