The starting potential of Geiger‐Mu¨ller counters filled with various pressures of air, helium, hydrogen, oxygen, argon, and two argon‐oxygen mixtures have been accurately determined for counters with oxidized copper cylinders 1.0 cm in diameter, and bare tungsten wires 0.0075 cm, 0.0125 cm, and 0.025 cm, diameter. Except at low pressures, Werner's equation fits the results fairly well, when suitable constants are chosen, for all these gases except helium. At low pressures, the starting potentialsdiminishas the size of the wire increases.