Methanol and ethanol have been identified as oil-soluble by-products generated by the aging of oil-impregnated cellulosic insulation materials of power transformers. Their presence provides useful information for diagnostics and end-of-life transformer estimation. Despite their value as cellulose degradation indicators, their sensitive and accurate determination is challenged by the complex oil matrix. To overcome this constraint, we present a simple, fast and direct procedure for their simultaneous determination in mineral insulating oil samples. The procedure uses a static headspace sampler coupled with a gas chromatograph equipped with a mass spectrometer. The selected method parameters permitted adequate separation of these two compounds from the complex oil matrix and quantification at ng g(-1) concentrations. An original internal standard procedure was developed, in which ethanol-d6 was added to all studied samples and blanks, with adequate resolution between the internal standard and its isotopomer ethanol. The method was validated in terms of accuracy and reproducibility for both analytes. The method detection limit, 4 ng g(-1) for methanol and ethanol, is well below the value (µg g(-1)) achieved by a standardized method for methanol determination in crude oil. During method validation studies, a relative error of approximately 6% was obtained for both methanol and ethanol with excellent reproducibility, average %RSD, below 2%. An experiment control chart, constructed to evaluate long-term reproducibility, indicate an overall good reproducibility (%RSD<3%) for 1000 ng g(-1) control solutions. The applicability of the method to the direct analysis of trace methanol and ethanol in oil from field transformer samples was successfully demonstrated. This analytical method is of high relevance to the electrical utilities as it allows indirectly assessment of the level of deterioration of the critical cellulose, an inaccessible part of a power transformer.