Oxyhydrides, in which oxide and hydride anions share the same anionic lattice, are relatively rare compounds. La2LiHO3 belongs to this family. We report the synthesis of La2LiHO3 by means of an alkali halide flux method, which allows the production of larger quantities of material relative to the usually adopted synthesis routes. Powder X-ray and neutron diffraction studies show that La2LiHO3 adopts an n = 1 Ruddlesden–Popper (RP)-type structure with an orthorhombic distortion (Immm) due to hydride and oxide anion ordering. No sign of polymorphism is observed. La2LiHO3 is seen to decompose in an oxygen atmosphere at ∼450 °C into La2LiO3.5. We show that the high mobility of hydride anions close to the decomposition temperature is likely the main factor in inducing the oxidation. The crystal structure of La2LiO3.5 is also determined and takes an n = 1 RP-type structure with an orthorhombic distortion (Fmmm). This newly reported large-scale synthesis approach, combined with the proven high thermal stability, is a key factor for potential practical applications of this oxyhydride in real devices.