This International Standard describes a method for the simultaneous measurement of conductance G and capacitance C enabling the calculation of the dielectric dissipation factor tan δ of insulating liquids. The proposed method applies both to unused insulating liquids and insulating liquids in service in transformers and in other electrical equipment. The standard is no substitute for IEC 60247; rather it complements it insofar as it is particularly suited to highly insulating liquids and it recommends a method of measurement for these liquids. This method allows values of the dielectric dissipation factor as low as 10–6 at power frequency to be determined with certainty. Moreover, the range of measurements of tan δ lies between 10–6 and 1 and can be extended up to 200 in particular conditions.