The energies of the α-particles from weak preparations of several new radioactive substances were measured in a grid ionization chamber filled with argon. The accurately known energies of α-particles from several well known sources were used for calibration. The voltage pulses produced by electron collection from the tracks were amplified in a linear amplifier of high stability and converted into flat-topped pulses. An accurately known voltage was subtracted, and the small residual pulses were further amplified. The frequency distribution of pulse sizes was recorded on an electronic pulse analyzer.The final values of the energies are as follows:92U233, 4.823 ± 0.003; 89Ac225, 5.801 ± 0.010; 87Fr221, 6.298 ± 0.010;85At217, 7.023 ± 0.010; 84Po213, 8.336 ± 0.005; and 94Pu239, 5.159 ± 0.005 Mev.The ionization–energy curve in argon was found to be linear within experimental error for α-particles of energy 5 to 9 Mev. Extrapolation of this line determines an intercept of 85 kev. at zero ionization.