An attempt is made to describe briefly the selection of oil crudes, and the choice of refinery methods best suited for the manufacture of various types of electrical oils, e.g. transformer and switch oils, and condenser and cable saturants. Methods of test are discussed and consideration is given to the changes undergone by these liquid dielectrics under high stresses. Highly paraffinic oils are shown to be undesirable in transformers, cables and condensers, and it follows that solvent processes are, in general, unsuited to the production of such oils. Rosin is shown to be a desirable constituent of cable saturants. It is not possible within the scope of this paper to give more than a small fraction of the available experimental evidence in proof of the statements made.