High vacuum pumping liquid and method

In a method of high vacuum pumping in an environment which contains chemically aggressive substances such as halogen or other Lewis acid, ammonia, or high proportion of oxygen, the pumping liquid comprises one or more esters of general formula I: R2C(CH2.O.CO.A)2 (I) in which formula each R is independently .CH3, .C2H5, .CH2.O.CO.A or .CH2.O.CH2.CR min (CH2.O.CO.A)2 where R min is .CH3, .C2H5 or .CH2.O.CO.A, and each A is independently alkyl (preferably tertiary alkyl) having 4 to 10 carbon atoms, together with 0.2% by weight of organic antioxidant having vapour pressure of the same order of magnitude as that of the ester. The esters used in the invention are preferably esters of pentaerythritol, trimethylolpropane, ditrimethylolpropane and dipentaerythritol. The esterifying acids may be straight or branched chain alkanoic acids but are preferably highly sterically hindered acids, such as pivalic acid (trimethylacetic acid) and 2,2-dimethylpentanoic acid. The antioxidant is preferably of the phenolic/amine mixed type containing amine and phenol in approximately equal stoichiometric amounts, for example 2% of alkylated N-phenyl-1-naphthylamine together with 0.5% of di-t-butyl-p-cresol ("butylated hydroxytoluene").