Test results were summarized in Table 1 and Figs. 4–7. It was found that parameter TAN (measured by titration) and parameters of Fluidscan show the oil degradation started in 550 h of oil oxidation test. Once oil oxidation was initiated, the acid number increased abruptly, as shown in Figs. 4 and 5. FTIR analysis of oil sludge samples formed at 822 h clearly showed a significant increase in the carbonyl peaks (AC@O, near 1700–1750 cm1) and hydroxide peaks (AOH, near 3300–3500 cm1), which are indications that the oil was oxidized, as shown in Fig. 6. Decrease of insulating property of the oxidized oil at 822 h is confirmed by the increase in dielectric loss tangent (from 0.001% to 2.55%). It increases with the introduction of polar molecules such as esters, aldehydes and ketones. As it is evident from UV–VIS spectra (Fig. 7), a prominent absorbance waveband 370–430 nm appeared in the spectrum of oxidized oil sample during 822 h. From the Table 1, we can see that optical densities Db of fresh and oxidized oil measured in range 400–510 nm are in close correlation with UV–VIS spectroscopy data. It is thought that obviously the absorption in this waveband can be used to monitor transformer oil condition particularly by on-line measurement of oil density.