Laboratory-scale NMR studies of transformer oils reveal the presence of highly branched aliphatic and aromatic hydrocarbons with evidence of proportional balance in both aged and new oils. The primary to secondary proton ratios of aged oils were determined to be approximately 1, whereas the ratio ranged from 1.120 to 1.194 in new oils. Analyses indicate that several transitions in the chemistry of oil components contributed to the observed composite peaks and did not allow the identification of specific compounds beyond their functional groups. Resonances of non-hydrocarbons are observed for concentrations between 2 and 4 ppm, while the aromatic families occur at concentrations between 9 and 6 ppm. No consistent trend is observed between oil aging and the evolution of oil chemical composition. However, the color numbers, 0.50-1.82, increase with the oil age and the direction of color change matches that of increasing turbidity from new (T34 and T44) to old oils (T14 and T24). This implies that turbidity may be a dependable method of indexing the aging of transformer oils. (C) 2010 Elsevier Ltd. All rights reserved.