This International Standard establishes the measurement of the interfacial tension between insulating liquid and water by means of the Du Noüy ring method close to equilibrium conditions. The interfacial tension of insulating liquids changes with time depending on the type and nature of the ageing products. This process is more pronounced with aged than with new insulating liquids. In order to obtain a value, which provides a realistic expression of the real interfacial tension, a measurement after a surface age of approximately 180 s is recorded.