The paper describes the experiments carried out during the last 20 years on the corrosion of steel-cored-aluminium overhead-line conductors on the Grid system. It is found that the average rate of deterioration, based on the reduction in mechanical strength, is 0.4–0.5% per year, and that the average service life of the conductor is 25–30 years. The rate of deterioration varies greatly in different localities, extreme values for the life of the conductor being from 3 to, say, 50 years.The mechanism of corrosion is described, and it is shown that there are two types of corrosion, one being associated with industrial districts and the other usually with coastal regions.The prevention of corrosion is discussed and information is given on service experience with greases and other protectives. Recommendations are made on methods of protecting conductors by treatment with a suitable grease.