We investigate the low temperature anion exchange behavior of hydride and oxide in perovskite oxynitrides. CaH₂ reduction of (Sr_1–xLa_x)Ti(O_3–xN_x) (0<x ≤ 1) resulted in the selective anion exchange of hydride for oxide rather than nitride, yielding the oxyhydride-nitride  (Sr_1–xLa_x)Ti(O_3–x–yH_yN_x). However, the exchange of hydride is drastically suppressed with increasing nitrogen content and is completely impeded when the nitride content reaches 10% of the anionic site. This implies that the N^3– anions in the oxide lattice play a crucial role in lowering diffusion of O^2–(and H^–). The present study indicates the necessity to consider kinetic aspects when manipulating anion compositions, in particular in a mixed anion system with a small amount of anion vacancies.