This technical report offers a test method for the classification of mineral insulating oils as either paraffinic or naphthenic, by means of low-temperature differential scanning calorimetry (DSC). For the purpose of this technical report, the typical operating temperature range extends from –100 °C to +100 °C. The temperature range can be extended, depending upon the instrumentation used. The method is applicable to mineral insulating oils obtained from petroleum crudes. It may be also applied to mineral oils containing pour point depressants, as these additives do not prevent the formation of paraffin crystals but only the growing of such micro-crystals.