Samples of 233U and of natural thorium have been irradiated in high neutron-flux facilities, in both soft and hard neutron spectra, and for both short and long exposure times. Included are exposures resulting in depletions of more than 90% of the 233U in the fissile material and burnups of more than 30 000 MWd/MT in the fertile material. Postirradiation mass analyses of the total and the isotopic uranium, of 137Cs, and of the neodymium isotopes generally agree within a few percent with corresponding calculations based on measured exposure histories. Reactivity measurements between irradiation cycles provide experimental results for the fissile content and fission-product poisoning as functions of both irradiation and cooling time. Corresponding results obtained from calculated concentrations agree with measurements to ∼1% for the fissile content and 3% for the effective one-group fission-product poison cross section. However, fission-product poison cross sections in two energy groups (thermal and epithermal) exhibit differences between measurement and calculation that are believed to be attributable to a lack of adequate information on important fission products in the literature. Experimental results for transient absorbers in irradiated 233U give at least 20 000 b for the neutron absorption resonance integral of 149Pm. This is a factor of 15 higher than that obtained by a 1/v extrapolation of the thermal cross section. For transient 135Xe, the measured absorption is 7.5% higher than that calculated using ENDF/B-IV data. Information is also provided concerning such matters as fission yields and neutron absorption of neodymium isotopes, the existence of significant transient fission-product poisons other than 135Xe and 149Sm, and the shielding of 233U by 232Th. Such shielding suggests the need for a change in the energy dependence of the 232Th thermal-neutron cross section.