Abstract Barium zirconium sulfide (BaZrS₃) is an earth-abundant and environmentally friendly chalcogenide perovskite with promising properties for various energy conversion applications. Recently, sulfurization of oxide precursors has been suggested as a viable solution for effective synthesis, especially from the perspective of circumventing the difficulty of handling alkali earth metals. In this work, we explore in detail the synthesis of BaZrS₃ from Ba-Zr-O oxide precursor films sulfurized at temperatures ranging from 700 °C to 1000 °C. We propose a formation mechanism of BaZrS₃ based on a two-step reaction involving an intermediate amorphization step of the BaZrO₃ crystalline phase. We show how the diffusion of sulfur (S) species in the film is the rate-limiting step of this reaction. The processing temperature plays a key role in determining the total fraction of conversion from oxide to sulfide phase at a constant flow rate of the sulfur-containing H₂S gas used as a reactant. Finally, we observe the formation of stoichiometric BaZrS₃ (1:1:3), even under Zr-rich precursor conditions, with the formation of ZrO₂ as a secondary phase. This marks BaZrS₃ quite unique among the other types of chalcogenides, such as chalcopyrites and kesterites, which can instead accommodate quite a large range of non-stoichiometric compositions. This work opens up a pathway for further optimization of the BaZrS₃ synthesis process, straightening the route towards future applications of this material.