Preferential dissolution of polyphase nuclear waste materials in short term leach tests can exaggerate radionuclide release rates when extrapolated to the lifetime of the waste form. Possible preferential leach phenomena are associated with the presence of cracks, intergranular phases and readily soluble phases. The rate of dissolution and the microstructural connectivity of the most soluble phase determine the period over which perferential dissolution is observable. The connectivity of phases is amenable to control during processing by altering the starting green density of the precursor powders.