In this study, improved observables and new endpoint criteria were developed to assess the remaining useful life of mineral transformer oil using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT-IR). The tested oil was exposed to thermal ageing at temperatures of 110, 120 and 130 degrees C for times ranging from 100 to 2400 h. The intensity of the infrared spectral band of the low-temperature inhibitor and the temperature of the thermo-oxidative DSC peak were considered for evaluation. The obtained results were correlated with standardized dielectric dissipation factor (tan delta) measurements. The results indicated that the analysed observables changed continuously during the applied thermal ageing. The endpoint criteria were subsequently determined and Arrhenius diagrams were constructed. The results demonstrated that the determination of the actual state or the lifetime of the transformer oil via DSC or FT-IR may be beneficial in terms of reducing the measurement time, the influence of moisture on the measurement results and the oil volume required for the tests. Thus, DSC and FT-IR represent promising alternatives to dissipation factor measurements. (C) 2016 Elsevier B.V. All rights reserved.