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Fifteen transformer oils used in commercial transformers (power 12.5 MVA, 25 MVA or 240 MVA) were monitored for 24 h to 276 h for humidity (water content), resistivity and dielectric loss coefficient by standard methods. The water content was mainly in the range 10 ppm to 20 ppm and showed a slightly increasing tendency (in the most of transformers) but never was higher than the allowable limits (30 ppm to 40 ppm). No close connection between the breakdown voltage, resistivity or dielectric loss coefficient and the water content was found. However, the longer the transformer operation was, the more intensive degradation of the oil was observed.