Resistivity change during isochronal aging of a high purity Fe-3.8 at.% Mo alloy is investigated. The resistivity first decreases between 300℃ and 400℃, passing through a minimum it increases to a maximum at 650℃ and decreases again. Molybdenum atoms are considered to form clusters in the first stage of resistivity decrease. The clustering at temperatures below 400℃ is possible only when quenched-in excess vacancies are present and enhance the solute atom diffusion. This observation implies that vacancies in pure iron also migrate at an appreciable rate at around 350℃ with an activation energy of about 1.2 eV
