The cross section for capture of two electrons by protons in single collisions with hydrogen and inert gas targets has been measured over the energy range 2 to 50 keV. Cross-section values are given relative to the well-known single-electron-capture cross section by protons in each target gas. In hydrogen, a peak value of 1.0×${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$/molecule at 17 keV is obtained for the double-capture cross section ${\sigma }_{1,-1}$. The double-peaked cross section versus velocity function, which is probably attributable to $\frac{10}{-12\mathrm{}}$ and $\frac{10}{-13\mathrm{}}e$ processes, is confirmed. In He, the cross section ${\sigma }_{1,-1}$ reaches a maximum of ${7.10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$/atom at 30 keV and steadily decreases to ${2.10}^{-20\mathrm{}}$ ${\mathrm{cm}}^2$/atom at 2 keV. In Ne, the present values show a peak cross section of 1.2×${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$/atom being reached at 14 keV. The present values when extrapolated above 50 keV agree well with Mittleman's corrected Born approximation calculations for ${\mathrm{H}}_2$.

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