Reaction cross section, σ, for the HeH⁺ + D → HD⁺ + He reaction as a function of the collision energy, E_r. The experimental results (black markers) are compared to our theoretical TIQM and QCT results. Also shown are TIQM and QCT calculations for HeH⁺ + H → H${}_2^{+}$ + He, which are compared to previous theoretical calculations that clearly deviate from both our experimental and theoretical results at low energies.