Extraction with N-methyl-2-pyrrolidone (NMP) via the three-stage mixer-settler operation was studied as possible process for re-refining of waste mineral insulating oil. The following extraction process parameters were investigated systematically in order to determine their optimum values: amount of water as co-solvent in NMP, extraction temperature and solvent/oil ratio. The process parameters and resulting oil chemical compositions were found to influence the electrical properties and the oxidation stability of the re-refined oil. The levels of aromatic, paraffinic and polycyclic aromatic hydrocarbons were determined using IR spectroscopy and HPLC, while other relevant chemical and electrical properties of waste and re-refined oil were determined according to IEC, ISO and ASTM standards. Extraction using NMP was found to be an adequate method for waste mineral insulating oil re-refining, according to the application properties of the re-refined oil. Moderate extraction temperature, 1% water in NMP and a low solvent/oil ratio (0.5) were determined to be the optimum process parameters. (C) 2006 Elsevier B.V. All rights reserved.