Using in situ methods to characterize the state of a system during reactions is critical to understanding and improving solvothermal syntheses. This work demonstrates the use of in situ energy-dispersive X-ray diffraction (EDXRD) to investigate the local dynamics during solvothermal formation of Cu₄O₃ using a general-purpose full-sized laboratory oven. This allows for direct comparison of in situ data with laboratory-based reactions. Using in situ EDXRD, changes in the local amounts of Cu₄O₃, Cu₂O and CuO within approximately 100 × 100 × 700 µm gauge volumes during solvothermal Cu₄O₃ formation were recorded. Fast conversion between Cu₂O and CuO was observed in the solvothermal environment, whereas Cu₄O₃ was found to be chemically stable against disturbances once formed. The observed differences in local dynamics give further support to the differences in formation mechanisms between Cu₄O₃ and Cu₂O/CuO proposed here.