The paper presents experimental investigations of the thermohydraulic phenomena of top vented blowdown processes of initially supercritical carbon dioxide. The initial fluid conditions were chosen such that flashing occurred after saturation conditions were reached. The investigations were focused on pressure and temperature transients. It was observed that the pressure at which flashing occurred first depends mainly on the initial fluid conditions due to the almost isentropic change of state during the supercritical/subcooled blowdown. Furthermore, void fraction profiles along the axis of the vessel were measured by means of a gamma densitometer. The void fraction profile is influenced strongly by phase separation effects. Various stages of characteristic void profile were observed.