The structural composition of fresh and aged (under model conditions) GK-grade transformer oil and petrolatum oil is studied by UV and 1 N NMR spectroscopy. Comparative experimental studies showed that sediment formation in transformer oil depends essentially on aromatic compounds rather than oxidized compounds, i.e. after reaching a certain concentration arenes coagulate forming solid particles that precipitate. It is concluded that in transformer oil, the aromatic hydrocarbons must be limited to the minimal content possible.