The breakdown potentials have been measured for argon and helium discharges using three di®erent cathode materials: aluminium, silver and magnesium. The measurements show that lower breakdown potentials are associated with lower work function of the cathode material. For the three di®erent cathodes, the sec- ondary ionization coe±cients have been estimated using the measured values of the breakdown potentials and the ¯rst ionization coe±cient, in the range 0.60 to 2.25 V/[Pa cm] (80 to 300 V/[torr cm]) for Ar discharge and in the range of 0.15 to 1.80 V/[Pa cm] (20 to 240 V/[torr cm]) for He discharge. The minimum breakdown potential has been found at (pd)min = 80 Pa cm (0.6 torr cm) in Ar discharge and at 530 Pa cm (4.0 torr cm) in He discharge.