A study has been undertaken to determine the effect of microstructural stability on the properties of ground state crystal structure of Ni4Mo with D1a superlattice. Thermal aging experiments have been carried out at 600, 700, and 800 °C for up to 1,000 h and the corresponding effects on room temperature tensile properties and corrosion resistance in boiling 20% HCl have been determined. Details of the ordered microstructure have been characterized using x-ray diffraction, optical metallography, scanning electron microscopy, transmission electron microscopy combined with energy dispersive x-ray spectroscopy, and Auger electron spectroscopy. It is shown that the D1a superlattice with nanoscale microstructure has a potentially useful combination of mechanical strength and corrosion resistance. However, changes in the microstructure of the D1a superlattice resulting from spontaneous recrystallization after extended aging are found to have adverse effects on tensile ductility and corrosion resistance. This behavior has been correlated with formation of Mo-depleted zones alongside grain boundaries. Although first-principle calculations can accurately predict the ground state structures of many materials, the results of the study show that these methods are still in a stage where some application relevant properties of complex materials, such as the D1a superlattice of Ni4Mo, cannot be predicted.