An apparatus for the vacuum pre-treatment of oils has been designed and constructed; the method of using it is described. An assessment has been made of the efficiency of the apparatus, which is capable of removing up to 1% added water in one or two treatments depending on the oil viscosity, with a consequent improvement in the electrical properties (d.c. resistivity and power factor) of the oils. Loss of volatile oil components has been shown to be very small, and oxidation of oil during treatment appears to be unlikely. The apparatus has been used to study the rates at which degasified oils of different types re-dissolve air. Under static conditions at 20° C, absorption in the initial stages is very rapid, becoming more gradual later. The rates of re-solution of air between one and six hours are in the inverse order to the viscosities of the four oils examined.