Transformer oils (TOs) at different degrees of degradation were studied by means of a kinetic method using, the model reaction of initiated cumene oxidation and by IR, ESR, and NMR spectroscopy. In the course of TO aging, the antioxidant additive ionol was consumed, and the concentration of new antioxidants, paramagnetic canters, and fused aromatic compounds increased. It was found that one of the main causes of the decrease in the dielectric properties of the oils (increase in the dielectric loss tangent) is the formation of free radicals inducing the formation of colloid structures.