Measurements have been made under impulse conditions of the coefficients A´, the number of H⁺ ions emitted per 250 keV H^- ion, and B´, the number of H^- ions emitted per 250 keV H⁺ ion, for metal surfaces covered with the contaminating layers likely to be formed in continuously-pumped high-voltage apparatus. The values obtained for A´ were 1.0, 1.1 and 0.54, and for B´ 0.43, 0.24 and 0.44 for copper, aluminium and steel targets respectively. The product of these coefficients is such as to make very probable the hypothesis that pulse discharge conduction in these systems is due to the regenerative exchange of positive and negative ions of hydrogen. The transient nature of this form of conduction is thought to be due to the charging up of the insulating contaminant.