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Additives to insulating oil and oil treatment by absorbents are investigated by thermal aging tests with the aim of suppressing flow electrification in power transformer insulation. 1, 2, 3-benzotriazol (BTA) is found to be effective for suppressing the increase of electrostatic charging tendency (ECT) in oils that contain either sulfide or sulfoxide compounds. Sulfide compounds are contained in new oil while sulfoxide compounds are generated by the oxidation of sulfide compounds. However, 2, 6-ditertiary-butyl paracresol (DBPC) is only effective for oils that contain sulfide compounds. The addition of sulfoxide compounds significantly increases the ECT of DBPC-added oil by the thermal aging tests. DBPC can suppress the conversion of sulfide to sulfoxide by blocking the generation of peroxide. However, DBPC has negligible effect on suppressing the increase in ECT by sulfonium ions, which are generated by the oxidation of sulfoxide. Negatively charged BTA ions, which are generated by the dissociation of hydrogen ions from BTA molecules, can mitigate the increase in ECT by sulfonium ions, which charge insulating oil positively. The effect of BTA is found to be lost when the concentration in oil decreases due to the adsorption on solid insulators. Oil treatment by absorbents such as clay, activated carbon and silica gel is found to remarkably decrease the ECT of deteriorated oil. However, additional thermal aging test increases the ECT to the same level as before the oil treatment. Although the additives and the oil treatment by absorbents are effective for suppressing and decreasing the ECT of insulating oils, their long-term effect is not assured. Thus periodical measurements of the ECT of insulating oils are crucial for the sake of preventive maintenance of aged transformers.