Since the introduction of SF6 in the 1950s, gas-insulated high voltage circuit breakers and Gas-insulated switchgear (GIS) have improved considerably, in particular concerning required drive energy for operation, compactness, and reliability. Nevertheless, high voltage insulation design has become increasingly challenging in recent years. Customer demands for reducing the physical footprint of HV equipment has led to an increase in operational field stress and therefore, much higher pressure on tolerances and increased susceptibility to defects. Dielectric design is based on how electrostatic fields are distributed and how much stress they can generate. In the course of conducting the intensive theoretical and experimental investigations on the dielectric design of insulation systems applied to high voltage power and pulsed power applications, it is becoming necessary to consider the influence of phenomena that have not been considered before. On top of that, as SF6 is one of the greenhouse gases listed in the Kyoto Protocol, SF6 usage regulations have been implemented in many industries. In Europe, SF6 is regulated under the F-Gas directive, which bans or restricts its use for several purposes. Several studies have indicated that CO2 is a viable alternative to SF6 for the transmission and distribution of electricity. In this context, this project aims to investigate a potential new phenomenon, the occurrence of secondary discharges caused by propagating streamer or leader discharges in HV gas insulation. Learning more about such phenomena can help us improve the dielectric design or perhaps explain the occurrence of breakdowns reported in high voltage equipment for which no obvious cause could be found. Specifically, CO2/O2 mixtures will be studied, and its results compared to SF6. Breakdown in gaseous insulation is caused by propagating discharges that are external in ambient air, e.g. on a bushing, or inside high-pressure insulation, e.g. across the surface of a GIS insulator. During the propagation of such discharges and flashovers (streamer or leader), transient electric field enhancement may occur, leading the local fields to exceed the inception fields at other locations. This can result in secondary discharges. Dielectric experiments using Image Intensifier and Photomultiplier tubes (PMT) have been conducted at three different pressure ranges; SF6 and CO2/O2 were selected as the insulation mediums. A negative polarity electric field is expected to trigger the secondary discharge. An analysis of images obtained from optical investigations and time lag records obtained from PMT signals suggested that secondary discharges can occur at low pressures (0.2MPa) in both SF6 and CO2/O2. At higher pressures (0.4 to 0.6MPa), no secondary discharges were detected. The reason for this is that, at higher pressures, the breakdown field is higher leading to a faster propagation of discharge across that gap. Hence, the formative time lag of the discharge is very short (some 100푛푠 or less) and this short time is not sufficient for secondary discharge inception.