The single-electron capture and loss cross sections ${\ensuremath{\sigma}}_{0,\ensuremath{-}1}$ and ${\ensuremath{\sigma}}_{01}$ for 2-50-keV hydrogen atoms incident upon hydrogen molecules and inert-gas atoms have been measured directly by observing the growth of the fast-collision products (i.e., the fast primary $\mathrm{H}_{1}^{}{}_{}{}^{0}$ which have changed charge) with the target-gas number density. In ${\mathrm{H}}_{2}$, He, Ne, and Ar, the present values of ${\ensuremath{\sigma}}_{0,\ensuremath{-}1}$ and ${\ensuremath{\sigma}}_{01}$ generally confirm previous measurements, with the notable exception that in ${\mathrm{H}}_{2}$ the low-energy values of ${\ensuremath{\sigma}}_{01}$ are as much as 30% smaller (at 2 keV) than the values of McClure. In Kr and Xe there are no previous measurements of ${\ensuremath{\sigma}}_{0,\ensuremath{-}1}$, while the only previously reported values of ${\ensuremath{\sigma}}_{01}$ are smaller than the present values by as much as a factor of 3. For a hydrogen-gas target, the present measurements do not show any large subsidiary peaks. The use of an electrostatic field, prior to the collision cell, to quench possible excited states in the primary $\mathrm{H}_{1}^{}{}_{}{}^{0}$ atom beam is shown to have only a very small effect upon the measured cross sections. Both ${\ensuremath{\sigma}}_{01}$ and ${\ensuremath{\sigma}}_{0,\ensuremath{-}1}$ are found to increase in value with increasing atomic number for all the inert gases. The low-energy (adiabatic) region is extended down to 2 keV, where it is seen that ${\ensuremath{\sigma}}_{01}$ rises exponentially with increasing relative velocity of the colliding particles.