In this work, the filament chemistry of uranium was investigated under conditions encountered in thermal ionisation mass spectrometry (TIMS), using a rhenium filament material without the addition of ionisation enhancers. The investigations were conducted using several analytical techniques such as X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and the modified-Knudsen Cell Mass Spectrometry (KCMS). The techniques provide useful information on the mechanisms associated with uranium evaporation and ionisation using a rhenium filament as surface ionisation source. The experimental results showed the sample behaviour to be consistent with the existence of a typical UO₂ phase forming on the filament surface. The composition of the resulting vapour phase is dominated by the presence of uranium oxide species, with a smaller fraction of uranium ions.