This article examines the visible transmission spectra of transformer oils (TOs) as indicators of their aging. While aging is traditionally associated with oxidation, the authors identify an alternative mechanism – the accumulation of aromatic compounds that degrade the oils“ performance. Using spectroscopy, the authors established a relationship between transmission spectra, molecular absorption, and Rayleigh scattering by dispersed particles formed during aging. A direct correlation was found between aging and changes in transparency caused by increasing concentrations of dispersed particles and aromatic compounds. These data are important for operational quality control of oils and predicting their replacement intervals.