This paper reports on the effect of ageing by free-burning arcs in 7.5% C5-fluoroketone (C5-FK) with 92.5% technical air in comparison to that in technical air (80% N2, 20% O2) at 1.3bar absolute pressure. The gases are aged by applying a series of arcs dissipating an accumulated energy of around 315kJ. It is found that the arc voltages in technical air and technical air with C5-FK are in the same range and do not vary significantly as a function of ageing or current amplitude (∼40-900 A). Contact erosion in both mediums is found to be similar if the discharge procedure is same. However, erosion increases significantly if ageing is performed in a short contact gap that needs more arcing operations to achieve similar level of arcing energy accumulation. Furthermore, gas decomposition by-products are analysed using gas chromatography coupled with mass-spectrometry.