Attention is drawn to the fact that the majority of the published work on insulating oils has reference to their use for transformers and switchgear. The requirements for cables are briefly outlined. Experimental results are then quoted showing the bearing of the physical and electrical characteristics on the performance of the finished cable. The high importance, both thermally and electrically, of avoiding gas pockets in the dielectric of a cable in operation is indicated, and the oil characteristics necessary to secure this are stated. A sensitive method of measuring changes in the electrical characteristics is described, and a possible application to sludging tests of transformer oil is suggested.