This paper focuses on the technology currently available for the production of severely refined, corrosive sulphur free base fluids used in the formulation of high quality transformer fluids. It will discuss the formulation, evaluation and performance characteristics of transformer fluids manufactured from corrosive sulphur free, iso-paraffinic base fluids. It has been shown that these fluids can pass the latest corrosive sulphur test methodology without the use of passivation additives (copper passivators). In addition, it will cover increased oxidative stability, heat transfer and biodegradability characteristics as compared to common naphthenic mineral oils.