With the increasing nonavailability of naphthenic crudes, attention is being turned to the manufacture of transformer oils from mixed-base Middle-East crudes, which incidentally happen to be rich in sulphur, and to present problems in producing oils with satisfactory electrical and oxidation characteristics. The Darius stocks, with relatively low sulphur plus CA(the proportion of aromatic carbon atoms in the oil molecule), possess excellent electrical properties while the oxidation stability needs upgrading. On the other hand the Arabian-mix stocks, with comparatively high sulphur plus CA, exhibit better oxidation stability. But since their electrical properties fall below the specified values, they require severe acid treatment which in turn adversely affects oxidation behaviour. In both cases, the oxidation stability can be improved by the use of antioxidants.