Relation of structure to magnetic susceptibility and internal friction is described for a 70% manganese‐30% copper alloy. The aged alloy exhibits a reversible fct to fcc transition at temperatures from 20° to 140°C, depending on the amount of aging. Magnetic susceptibility apparently obeys the Curie‐Weiss law but a change is observed in the temperature‐susceptibility curve at the structure transition temperature. The low‐stress internal friction of this alloy shows a resonance peak at the temperature of the structure change.Aging experiments with the 70 and 80% manganese alloys give evidence that quenching of alloys containing more than 80% manganese from solid‐solution temperatures does not prevent changes typical of alloys aged to initiate precipitation of alpha manganese. Thus the fct&rlarr2;fcc transformation that others have associated with the disappearance of antiferromagnetic ordering also is associated with an early stage of the alpha‐manganese precipitation.