1. Molybdenum hardens the solid solution after quenching and the higher the concentration of molybdenum the harder the solid solution. 2. After prolonged aging (up to 20 h at 680°C) the hardness increases, with simultaneous decrease in microhardness, which is due to the precipitation of intermetallic compounds of nickel and molybdenum. 3. A relatively short aging period (up to 4 h) does not change the lattice constant or the specific electrical resistance, which indicates that no changes occur in the alloyed solid solution. 4. The change in hardness after a short period of aging is probably the results of intragranular processesredistribution of alloyed elements and possibly a change in the dislocation density.