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Ester oils have been successfully used in distribution transformers for many years as satisfactory substitutes for mineral oil. In high voltage transformers however their application has been much slower partly due to the fact that the high viscosity of ester oil and hence uncertainties in efficient impregnation of cellulose insulation. In this paper the characteristics of the impregnation of cellulose insulation by ester oils are assessed and compared with mineral oil. Studies are carried out on thick laminated pressboard blocks, typical of support insulation structures in high voltage transformers. The degree of oil impregnation with time at various temperatures is examined. It is shown that the factors governing the impregnation processes with ester and mineral oils are very similar, i.e. mainly viscosities and surface tensions, and if a higher impregnation temperature is used for ester oils the same degree of oil impregnation can be achieved as with mineral oils.