Naphthenic and paraffinic oils were analyzed by modulated differential scanning calorimetry (MDSC). The results showed several improvements in the analysis of thermal properties when compared with standard DSC. The glass transition temperature (T-g), the enthalpy relaxation at T-g and the melting endotherms could be deconvoluted, and reversible melting could be identified. This allowed for an easier interpretation of the thermal properties of the oils. With MDSC, the T(g)s in mineral oils were found to coincide with endothermic enthalpy relaxation, which is generally regarded as a melting endotherm with standard DSC. A decrease in heat capacity after T-g was attributed to the existence of rigid amorphous material. From Delta cp at T-g and the oil molecular weight, the number of repeat units in the oil chains was estimated at less than 20, The T-g of a hypothetical pure aromatic oil was found to be similar to that for petroleum asphaltenes, and that for a naphthenic oil of infinite Molecular weight to be similar to that of petroleum resins. Crown Copyright (C) 2005 Published by Elsevier B.V. All rights reserved.