Studies are reported on the atomic and oxide ion formation of Pu and U during thermal ionization mass spectrometry. Different synthetic mixtures with Pu/U amount ratios of 2–10 were used to study the vaporization and ionization characteristics under different heating currents of vaporization and ionization filaments using a double filament assembly. The different ions observed were U⁺, Pu⁺, UO⁺, PuO⁺ and two different sets of filament heating conditions were identified to maximize the production of UO⁺ and Pu⁺ ions. It is observed that by using interfering element correction methodology and employing suitable filament heating currents, there exists a potential of determining ²³⁸Pu in Pu samples by TIMS with accurate accounting of isobaric interference from ²³⁸U.