The effectiveness of cataphoretic purification of neon was investigated in conjunction with work on breakdown potentials in neon below the Paschen minimum. The extreme sensitivity, resulting from the Penning effect, of the breakdown potential in neon to the presence of traces of argon was used as an indicator of the degree of purity of the neon. Under the conditions of this experiment the limit of sensitivity for argon was about 1:107. With a discharge current of 125 mA in a helically wound Pyrex tube 1.6‐cm o.d. and 2 m long, original concentrations of argon in neon of 50 ppm and 2%±1% were decreased to ≤0.1 and ≤1 ppm, respectively.