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The aim of this paper is the study of various parameters affecting the ageing of transformer oil in transformers of 150/20 kV. 44 samples of oil were taken from such transformers. The transformers function in the major Athens area, Greece. Parameters, such as breakdown strength (measured with a Foster test cell, according to IEC 156/95), oil colour (with a chromometer than meets the specifications PIN 51517-ASTM P155), humidity (measured with the Karl-Fischer method, according to IEC 814), interfacial tension (which is of importance especially in the early stages of ageing, measured according to ASTM D971-91) and tanδ (with a system counting tanδ values with maximum accuracy from 0.00001 to 4.0, measured according to IEC 247) were taken into account. Transformer ageing is strictly related to the rate of ageing of its whole insulation, and mainly of the oil. Thermal and mechanical stresses have as a result the oxidation of oil and the degradation of its insulating and cooling properties. Humidity and foreign particles also consist factors contributing to the decrease of oil dielectric strength and its premature failure. Foreign particles combined with humidity result in a decrease of interfacial tension of the oil. A colour index close to 3 is not necessarily an objective indication regarding the quality of the oil. One of the main conclusions is that all aforementioned measured parameters should be taken into account. One should not base his judgment about the quality of oil on only one or two of the above parameters. Special attention is given to the way one gets the oil samples. In case of contradictory results, a repetition of sampling and the corresponding parameter measurements are required. The frequent and careful oil sampling in such transformers is recommended in order to avoid malfunctions. In general, from the transformers investigated in the context of this work, in only very few cases oil filtering was required and none of the transforme- - rs was found to function with oil requiring replacement.