Abstract Metal hydride oxides are an interesting class of compounds with potential for hydride ion conduction and as host materials for luminescence. SmHO and HoHO were prepared from mixtures of the sesquioxides Ln ₂O₃ and the hydrides LnH_{2+ x} at 1173 K as gray powders (Ln=Sm, Ho). They crystallize in a fluorite type crystal structure with disordered anion distribution (Fm3̅m; SmHO: a=5.46953(6) Å, V=163.625(5) ų; HoHO: a=5.27782(3) Å, V=147.016(2) ų, based on powder X-ray diffraction) and show stability towards air. Lanthanide-oxygen and -hydrogen distances are 2.36838(3) Å in SmHO and 2.28536(1) Å in HoHO and comparable to those in binary lanthanide oxides and hydrides. Elemental analyses confirm the composition LnHO. Quantum-mechanical calculations show a negative enthalpy for the reaction RE ₂O₃+REH₃→3 REHO for all lanthanides and Y, with increasing values for decreasing ionic radii.