This paper reports volt-time characteristics of air and air mixture with 1% SF6 additive for a 1.25 5.2 cm coaxial-cylinder gap. The surge voltages used included waveshapes of 0. 260 Ps, 1.25660 ps impulse voltages as well as 1511200 ps, 275/2500 , us switching surge voltages. The experimental pressure ranged from 100 to 500 kPa (absolute) Additional tests were conducted using rod-shpere gaps in dry air and nitrogen. For steep-fronted impulses the breakdown values decreased with increasing the rate of rise of voltage. The effect depended upon the polarity of the voltage, the pressure, and the nature of the gas.