The aging behavior of an Fe78B13Si9amorphous metal alloy (METGLAS®2605S-2) for 60 HZ Application is examined in a nitrogen atmosphere in the temperature range 250°-375°C by monitoring changes in 1.4T power loss and exciting power and D.C. Hc. Extrapolation of the data to low temperatures (<250°C) is found to overestimate the magnitude of the change in magnetic properties. Analysis of the experimental activation energies and detailed structural and surface characterization reveals two distinct stages of aging, both originating primarily from B and Si oxidation at the ribbon surface. The initial stage is reversible and is associated with surface oxide formation. The later stage is irreversible and corresponds to α - Fe crystallization stemming from extensive metalloid oxidation.