The first molten plutonium reactor experiment (LAMPRE I) uses a liquid fuel alloy of plutonium and iron contained in small test tube shaped capsules of tantalum and cooled by liquid sodium. The development of compatible materials for the fuel, container, and coolant has been an important phase of the experiment. This paper reports on the methods of corrosion testing (developed for the work) and the results of experiments on the composition of the fuel. Both mass transfer attack and intergranular corrosion were found to be significant. Mass transfer was found to be controllable through the use of fuel additives which presumably formed protective layers on the tantalum. Intergranular corrosion was found to be strongly influenced by fuel composition and by container properties. All levels of calcium and magnesium in the fuel were found to be detrimental. Additives which were effective in minimizing mass transfer were found to be not always effective against intergranular attack. Testing methods included tracer techniques, radioautography, chemical analysis, and bend testing.