High energy ion scattering (HEIS) is used to study the clean and hydrogen induced surface reconstructions on Mo(100) at low temperature (80 and 10 K). In conjunction with computer modeling, lateral displacements of 0.12±0.02 Å along the 〈11〉 direction and a first‐layer inward relaxation of 0.11±0.03 Å for the clean reconstruction are measured. It is also established that the hydrogen saturated surface is bulk‐like. In addition the HEIS results, combined with new low‐energy electron diffraction data, indicate that even at temperatures as low as 10 K, hydrogen dissociates and diffuses on the surface and, at saturation coverage, quenches the reconstruction.