The current methodology for perovskite oxynitride synthesis is a gassolid reaction with ammonia. However, a competing reaction exists, where ammonia dissociates to inert N2, rather than a nascent, active nitriding species. Therefore, oxynitride synthesis is controlled by the composition of the gas phase. Control of key processing variables (temperature, flow velocity, sample placement, temperature distribution, furnace configuration) determines the gas-phase components (NH3, N2, and H2) and subsequently, the reaction rate and purity of BaTaO2N powders.