alternatives to the potent greenhouse gas SF6 in medium voltage (MV) gas insulated switchgear. MV products using this gas mixture are already commercially available, but there is still a lack of knowledge about this gas mixture especially regarding pre-breakdown phenomena. In this study, positive and negative streamers are studied in technical air with 7.5 %vol C5-FK at 1.0, 1.3, and 1.5 bar (absolute pressure). Similar tests were done in technical air (80% N2, 20% O2) for comparison with the C5-FK results. The streamers emerged from a grounded needle electrode placed 53mm below a planar electrode. The electrode setup was stressed by a non-standard lightning impulse voltage with voltage peaks varying from 52 to 129 kV. A photomultiplier tube (PMT) was used to register the streamers, and an iCCD camera was used to image the streamers. The pre-breakdown phenomena in C5-FK+air were found to be shorter, less than 1/5 of the length, and to branch less than in pure technical air. The streamer branches in both gas mixtures became wider and less clear when the pressure was decreased. Leader-like channels were observed for C5-FK+air at 1.5 bar (both polarities) and at 1.3 bar (negative streamers).