The alkaline sodium sulphite technique is one of the most successful for the treatment of archaeological iron but the mechanism by which it operates is rather different from that which has been assumed up to now. The chemistry of magnetite formation under reducing conditions is discussed, and experiments designed to confirm the mechanism are described. While treatment of freshly excavated material is most effective, the reason for the failure of the tech nique when artifacts have been allowed to dry out is explained. -- AATA -- AATA