The Jesse effect — the effect of common gaseous impurities on the ionization yield in noble gases, — was investigated as a function of the impurity concentration. Experiments were conducted using a specially constructed parallel-plate ionization chamber. A solid 63Ni, pure-beta emitter was used as the radiation source. Results showed a sharp increase in the ionization yield in He at low impurity concentrations (<∼1000 ppm) followed by a plateau at high concentrations (>∼4000 ppm) for all common impurity gases investigated. At the plateau, the ionization yield was found to be approximately 40–50% higher than the value obtained with ultra-high-pure He. According to these data, the magnitude of the tritium concentration measurement error associated with ionization chambers can be as high as 40–50% if common impurities are present in the He carrier. Since tritium itself is a Jesse effect-causing impurity in He, the calibration factor for a given ionization chamber will depend on the tritium concentration in He up to ∼350 TBq/m3. Only small changes in the ionization yield were observed for Ar. Therefore, common impurities will not impede the measurement of tritium in Ar with ionization chambers.