Studies are reported on the formation of atomic (M⁺) and molecular (MO⁺) ions of thorium and uranium in thermal ionization mass spectrometry. The studies were performed by using different synthetic mixtures with Th/U amount ratios as 10. 1 and 0.1. The results obtained on the formation of atomic and molecular ions from these synthetic mixtures employing high purity rhenium double filament assemblies are presented. It was observed that the presence of Th on the filament during TIMS analysis of U enhances the stability of UO⁺ ion. Further. ThO⁺ ion is ideal for the isotopic analysis of Th by TIMS whereas U⁺ or UO⁺ ion may be used for the isotopic analysis of U. These studies will be extremely useful to optimize the filament heating parameters for the simultaneous/sequential isotopic analysis of Th and U by TIMS. The optimized conditions of analysis were employed for the determination of ²³²U in a synthetic U sample containing traces of ²³²Th.