After an Introduction setting out the general problem of atmospheric pollution in relation to transmission-line insulators, the authors deal in Part I with the development of a saline-deposit test for such insulators. The production in the laboratory of wind and of salt spray is discussed, and a comparison is drawn between natural and artificial dew formation. Means of judging the severity of dew formation and of deposit formation are described, and standard tests outlined. An account is given of the various types of insulators tested and of their behaviour under test.Part II of the paper is devoted to an investigation of the possibility of protecting insulator surfaces from industrial and saline deposits. The protective action of a cylinder, and of a cylinder and disc in combination, are examined, and a description follows of the design and performance of a 33-kV insulator embodying a cylinder and a disc.In Part III the authors describe an inquiry into the formation of industrial deposits under natural conditions. Visual observations have been made of the deposit formation on insulators hung from a gantry on the roof of a laboratory in East London, and the leakage-current variations due to changes in weather conditions have been measured by a special form of recorder. The test-results obtained on different insulator types are compared, and some interesting conclusions are drawn from the discharges observed on the insulators during bad weather.