Numerous applications require the precise analysis of U isotope relative enrichment in sample amounts in the sub-nanogram to picogram range, among those are nuclear forensics, nuclear safeguards, environmental survey and geosciences. However, conven-tional thermal ionization mass spectrometry (TIMS) yields U combined ionization and transmission efficiencies (i.e ratio of ions detected to sample atoms loaded) less than 0.1 or 2% depending on the loading protocol, motivating the development of sources capable of enhancing ionization. The new prototype cavity source TIMS at ETH offers improvements from 4 to 15 times in com-bined ionization and transmission efficiency compared to conventional TIMS, yielding up to 5.6 % combined efficiency. Uranium isotope ratios have been determined on reference standards in the 100 pg range bound to ion-exchange or extraction resin beads. For natural U standards, n(235U)/n(238U) ratios are measured to relative external precisions of 0.5 to 1.0 % (2RSD, 2<n<11, conven-tional source) or 2.0 % (2RSD, n=6, cavity source) and accuracies of 0.2 to 0.7 % (conventional source) or 0.4-0.9 % (cavity source). Meanwhile, n(234U)/n(238U) ratios are determined to relative external precisions of 1.7 to 3.6 % (2RSD, 2<n<11, conven-tional source) or 5.6 % (2RSD, n=6, cavity source) and accuracies of 0.1 to 2.5% (conventional source) or 0.5 to 8.3% (cavity source), which would benefit further from in-run organic interference and peak tailing corrections.