Currently, preventive diagnostics of internal isolation in high-voltage equipment is of great importance for timely detection of defects and making the right decision to eliminate them. One of the main components of insulation are transformer oils of different brands. The article gives an overview of the chemical composition of mineral insulating oils established by various studies. In this paper, we present the results of studying the aging products of the most used brands of transformer oils (GK, TKp, T-750, Nytro 11GX, Nytro 10X) used in real operating conditions of electric grid equipment. Oxygen-containing compounds, saturated hydrocarbons, saturated and unsaturated carbocyclic hydrocarbons, as well as derivatives of naphthalene and anthracene series were identified by gas chromatography with mass-selective detection. An explanation of their origin is given. Attention is drawn to the fact that some compounds of liquid mineral dielectrics can be indicators for assessing the state of the «oil-cellulose» insulation system in high-voltage equipment.