In this study we investigate the topotactical deuteration of LaFeSi using in-situ neutron powder diffraction, yielding superconducting LaFeSi(H,D). The structural changes are linked to the thermodynamic nature of the chemical process, investigated using thermogravimetric analysis, differential thermal analysis, and mass spectroscopy. Pressure-composition-isotherm measurements further show a remarkably low equilibrium pressure. The reaction is found to be binary with two separate phases LaFeSi and LaFeSiD coexisting during the reaction. The reaction enthalpy, determined using differential thermal analysis is found to be ΔH = − 54.5 kJ/mol. Furthermore the anisotropy of the thermal expansion is observed to change significantly as αc∕αa changes from 6.6 to 1.2. All the LaFeSi(H,D) obtained samples were found to be superconducting with Tc ∼ 10 K, i.e. the value corresponding to stoichiometric composition. We conclude that this gas-solid process does not produce a solid solution with a tunable x(H), LaFeSiHx, where Tc would be continuously changed.