The environmental stability of CuBiSeCl2 was characterised to understand the powder degradation over time with exposure to air and moisture. CuBiSeCl2 powder was found to be stable in ambient air for over a month (S31). After mixing with water, depositing onto a glass slide and drying for 24 hours, it was observed that the black powder underwent a slight colour change to pink around the very edges. Comparison of PXRD data from before and after water exposure reveals the appearance of small impurity peaks, consistent with the formation of BiOCl (S32).