The electrometallurgical treatment of zirconium-based and Zircaloy-clad spent nuclear fuels will yield a metal waste form. The baseline composition for the waste form is zirconium-8 wt% stainless steel (Zr-8SS). The microstructure of the Zr-8SS alloy has been studied by scanning electron microscopy, energy dispersive spectroscopy, and neutron diffraction. The phases present in the as-cast alloy include Zr(alpha), Zr-3(Fe,Ni), Zr-2(Fe,Ni), Zr-2(Fe,Cr), and Zr(Fe,Cr)(2); a solidification sequence has been proposed to explain the formation and morphology of these phases. Alloy phase stability has been studied by thermal aging at 780 degreesC for periods up to 30 days. The phase changes that occur during thermal aging include an increase in Zr-3(Fe,Ni) and a decrease in Zr-2(Fe,Ni) content; reaction mechanisms have been proposed to explain these changes. The lattice parameters of alloy phases have been determined by neutron diffraction and found to be in agreement with those previously reported for similar phases. This study of alloy microstructures is the first step towards understanding the actinide and fission product distribution and predicting the corrosion behavior of the Zr-8SS metal waste form.