Under the sponsorship of the European Community, a group of 6 European research institutions has used state-of-the-art computer programmes and JEF2.2 data to evaluate the mass balances of MOX fuel pins after high-burnup irradiations in three thermal reactors, the PWRs of Saint-Laurent B1, Beznau-1 and BR3. The prediction of these mass balances is satisfactory for Pu isotopes (except Pu238) and Am241, while the other Am isotopes and the curium isotopes were underestimated by 7 to 30%. In general, similar trends are found by CEA with APOLL02 and by Belgonucleaire with WIMS7. The fuel burnup in these irradiations ranged from 10 GWd/t to nearly 100 GWd/t (at the hot spot in BR3). No bias is observed with respect to burnup. In addition, the quality of prediction was about the same in standard lattice PWRs and in overmoderated neutron spectra (BR3). In parallel, a critical review of the basic data files (JEF2.2 versus ENDF/BVI and JENDL3.2) was carried out for 3 isotopes: Am241, Pu240 and Pu242. The impact of the americium and curium underpredictions on some recycling scenario calculations was investigated. While the prediction of waste toxicity is good far a single MOX recycling step, the underestimates and deviations found above for the americium and curium masses imply an underestimation of waste toxicity by about 10% in the case of multiple recycling of actinides. This underestimation would largely increase in the case of dedicated actinide burners.