1. Molybdenum decreases the temperature interval in which martensite is formed in Fe+20% Ni alloys, while cobalt increases it. The additional elements investigated here affect the temperature interval of the martensitic transformation in the same way as that in steel. 2. Since the alloys must have a martensitic structure before aging (the effect of aging is due to the processes occurring in the α-phase), the amount of molybdenum—which sharply decreases the temperature of the martensitic points—should be limited to 3% (if the alloys are not subjected to cold temperatures). In the presence of cobalt—an element increasing the temperature of the martensite points—the amount of molybdenum can be increased to 5%. The positive role of cobalt in alloys of this type is probably due to the fact that the addition of cobalt makes it possible to increase the amount of molybdenum. 3. The hardness of annealed (unaged) alloys is independent of the ratio between austenite and martensite. Apparently, the hardness of carbon-free and unaged martensite is practically the same as that of the initial austenite. 4. The hardness of the alloys investigated cannot be used as a criterion of their degree of quenching.