The phase mixture CsPb2Br5/CsPbBr3 has raised interest as a promising material system for light emission, light detection, and even for photovoltaic devices owing to its high luminescence yield and improved device performance when compared with pure CsPbBr3 devices. Given that CsPb2Br5 and CsPbBr3 exhibit similar formation enthalpies, thermal treatment can induce phase transformations between them. The influence of that transformation on the optoelectronic properties is still unclear. In the present work, we report the effect of elevated-temperature (>580 K) postdeposition annealing on the luminescence of a mixed-phase CsPb2Br5/CsPbBr3 thin film deposited by coevaporation. Using combined photoluminescence spectroscopy, X-ray diffraction, and scanning electron microscopy techniques, we found that deep defects at the CsPb2Br5/CsPbBr3 interface contribute to quench the CsPbBr3 green luminescence and that annealing increases the defect concentration in CsPb2Br5, leading to a strong decrease of the external photoluminescence quantum yield. Our results do not confirm any passivating effect of CsPb2Br5—as-deposited or induced by annealing—as reported by other authors.