In the accelerated aging test of alkyl benzene with sulfur compounds, remarkable increases of the ECT were not detected for thiophene compounds, which are the major component of sulfur compounds contained in mineral insulating oil. However, some sulfide and sulfoxide compounds increased the ECT significantly. The ECT with reverse polarity was detected by sulfonic acid compounds. The molecular weight and structure of sulfur compound influence the increase of the ECT. The aging properties of ECT are influenced by the accumulation of various effects of these sulfur compounds, all of which are produced by the oxidation of sulfide compounds.