AbstractChemical bath deposition (CBD) is widely used to deposit tin oxide (SnO_x) as an electron‐transport layer in perovskite solar cells (PSCs). The conventional recipe uses thioglycolic acid (TGA) to facilitate attachments of SnO_x particles onto the substrate. However, nonvolatile TGA is reported to harm the operational stability of PSCs. In this work, a volatile oxalic acid (OA) is introduced as an alternative to TGA. OA, a dicarboxylic acid, functions as a chemical linker for the nucleation and attachment of particles to the substrate in the chemical bath. Moreover, OA can be readily removed through thermal annealing followed by a mild H₂O₂ treatment, as shown by FTIR measurements. Synergistically, the mild H₂O₂ treatment selectively oxidizes the surface of the SnO_x layer, minimizing nonradiative interface carrier recombination. EELS (electron‐energy‐loss spectroscopy) confirms that the SnO_x surface is dominated by Sn⁴⁺, while the bulk is a mixture of Sn²⁺ and Sn⁴⁺. This rational design of a CBD SnO_x layer leads to devices with T₈₅ ≈1500 h, a significant improvement over the TGA‐based device with T₈₀ ≈250 h. The champion device reached a power conversion efficiency of 24.6%. This work offers a rationale for optimizing the complex parameter space of CBD SnO_x to achieve efficient and stable PSCs.