In synthesizing mixed anion oxides, direct syntheses have often been employed, usually involving high temperature and occasionally high pressure. Compared with these methods, here we show how the use of a titanium perovskite oxyhydride (BaTiO_2.5H_0.5) as a starting material enables new multistep low temperature topochemical routes to access mixed anion compounds. Similar to labile ligands in inorganic complexes, the lability of H^– provides the necessary reactivity for syntheses, leading to reactions and products previously difficult to obtain. For example, BaTiO_2.5N_0.2 can be prepared with the otherwise inert N₂ gas at 400-600 °C, in marked contrast with currently available oxynitride synthetic routes. F^–/H^– exchange can also be accomplished at 150 °C, yielding the oxyhydride-fluoride BaTi(O, H, F)₃. For BaTiO_2.4D_0.3F_0.3, we find evidence that further anionic exchange with OD^– yields BaTiO_2.4(D^–)_0.26(OD^–)_0.34, which implies stable coexistence of H⁺ and H^– at ambient conditions. Such an arrangement is thermodynamically unstable and would be difficult to realize otherwise. These results show that the labile nature of hydride imparts reactivity to oxide hosts, enabling it to participate in new multistep reactions and form new materials.