THERE has long been a need for a diffusion pump oil which, while hot, would withstand atmospheric pressure without bad effects. G. P. Brown (Rev, Sci. Inst., 16, 316 ; Nov. 1945) has found that certain members of the family of silicones are stable under the above conditions and also have vapour pressures as low as, or lower than, the best commercial oils available. Comparative performances of a straight hydrocarbon oil ('Litton C'), an ester (Octoil'), a chlorinated aromatic hydrocarbon oil ('Narcoil') and two typical silicones in a 6-in. non-fractionating unbaffled all-metal diffusion pump are presented. Tests show that the highest vacuum is produced by the high-boiling silicone (b.p. 430° C.) and that the silicone is nearly completely resistant to oxidation when exposed to air while hot.