1. The effect of alloying iron and nickel with molybdenum has been studied, and it has been shown that the Mo concentration converting the alloys from one-phase to two-phase structures, hardens them over the whole temperature range studied. 2. Up to 700°C (1290°F) there is precipitation hardening in the two-phase region, which greatly increases hardness. 3. The long-time hardness of the nickel alloy with 25% molybdenum is higher at 600 and 700 than at 5.00°C (1110, 1290 and 930°F); the differences in the 30 and 3000-second hardnesses characterizing the creep tendency of the alloy, are approximately the same at 500. 600 and 700°C. 4. Increase of the test temperature to 800 and 900°C (1470 and 1650°F) shows that the 25% molybdenum alloy (Ni-base) softens more readily than the alloy with 7% Mo. 5. At 1000°C (1830°F) nickel-base alloys suffer identical softening irrespective of their Mo contents, and the hardness/time curve is similar to that obtained at 500–600°C. 6. Increase of the Mo content within the range studied only slightly affects the elastic modulus, which continuously decreases with increase of test temperature. 7. The logarithmic decrement of elastic vibrations does not change up to 500°C (930°F) in any of the alloys studied, but with further increase of test temperature, there is a sharp increase in internal friction; this process starts in one-phase alloys at a lower temperature than in two-phase alloys.