Figure 15 summarizes the test results of alkyl benzene with organic acid compounds. Remarkable increases of the ECT were not detected. Considering the results that little increase of the ECT was detected in the deteriorated acceleration test of alkyl benzene as shown in Figure 1, hydrocarbon and its oxide compounds have only a small influence on increases of the ECT of mineral insulating oil. Some sulfoxide and sulfonic acid compounds increase the ECT by heating. However, the ECT of alkyl benzene with the addition of various compounds in the room-temperature test was low. Considering the results that sulfoxide compounds caused the increases of ECT and that sulfonic acid compounds caused the increases of ECT with reverse polarity, ionic compounds are considered to be produced via sulfoxide compounds and influence the increase of the ECT of mineral insulating oil.