The invention relates to a high performance dielectric oil comprising approximately 75 - 95 % napthenic oil by volume and approximately 5 - 25 % ester oil by volume, particularly a synthetic ester oil and, more preferably, an oil of the polyolester family. Applications: high voltage electrical equipment, i.e. power, measuring, distribution or traction transformers, tap changers, bushings, distributors, oil switch circuit breakers, power capacitors, cables.