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The deposition of copper sulphide within oil filled high voltage equipment is the symptom of the use of sulphurous oil. In some cases, severe copper sulphide contamination has resulted in expensive equipment failure. It has been shown that sealed equipment, containing sulphurous oils and operating at elevated temperatures, are the more susceptible to the copper sulphide deposits. Recent work has suggested a mechanism for the deposits in the paper covering the copper conductors. Other work has suggested that the deposition may be the result of thermally driven gassing. However, the overall mechanism of copper sulphide formation and its deposition on the copper is still a topic of interest. The problem is complicated by the lack of a complete chemical characterisation of the various industrially available oils. A better understanding of the presence of the sulphur containing chemical species present in the oil and their thermal dependence will give a clearer insight into the mechanism of copper sulphide formation. This may ultimately lead to improved preventative methodologies which can be employed in the future. This paper will detail results from the use of various chemical analysis techniques applied to oil known to cause copper sulphide deposits. The results from X-ray fluorescence, UV/Vis spectroscopy, gas chromatography mass spectrometry and scanning electron microscopy are used in conjunction to reveal how the sulphur and copper concentrations change with the depletion of corrosive action.