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The aim of this paper is the study of various parameters affecting the ageing of transformer oil in distribution transformers of 20/0.4 kV. 69 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 DIN 51517-ASTM 155), 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 the whole insulating system, and mainly of the oil. Absence of transpiring system with silica gel has as a result the increase of oil humidity. The lengthy use of transformers under heavy load, and consequently under high temperatures, is a main factor for oil ageing (and oxidation). Especially in the last few years, the increased loads required by the transformers (which have to do also with the climatic changes), have as a result the additional stressing of the oil. Various arcs, resulting from short circuits in the network, have as a consequence the production of gases and sludge. Such gases and sludge influence in a negative way the insulating properties of the oil as well as its rate of ageing. Special attention has to be paid to the way one gets the oil samples. In the case the results are contradictory, one has to repeat the sampling and the corresponding measurements. Some comments are also offered regarding the manufacturers, who try to reduce the production cost of the transformers and their respective materials by reducing the size of the transformers. This on one hand has as a result the possibly higher breakdown stren- th of the oil, on the other hand, however, has as a result the faster increase of the oil temperature, and consequently the faster ageing and stressing of the mechanical parts of the transformer.