This work examines the capabilities of simulation codes to predict the concentration of nuclides in spent reactor fuel, in particular mixed-oxide (MOX) fuel, via comparisons with destructive radiochemical analyses performed on irradiated samples. We report on three MOX samples irradiated in a pressurized water reactor (PWR) and two UO2 samples irradiated in a different PWR. Actinide and fission-product concentrations were measured and were compared with concentration values obtained from simulation studies. The actinides include isotopes of uranium, neptunium, plutonium, americium, and curium. The fission products include isotopes of cesium, neodymium, samarium, europium, and gadolinium as well as 90Sr, 95Mo, 99Tc, 101Ru, 106Ru, 103Rh, 109Ag, 125Sb, 129I, and 144Ce. For many of the actinides, the predictions are quite good when compared with the measured values; but concentrations of some tend to be overpredicted. The cesium and neodymium, and some samarium concentrations, are well predicted, but some of the other fission products show variable results. The sensitivity of some of the results to sample-burnup estimates is discussed.