The　composition　of　SrCuxO　mixed　metal　oxides　(MMOs)　was　engineered　via　varying　the　amount　of　copper　relative　to　strontium.　As-synthesized　SrCuxO　were　highly　active　for　degrading　methyl　orange　(MO)　pollutant　at　dark　ambient　conditions　without　the　aid　of　other　reagents.　The　catalytic　activity　of　SrCuxO　demonstrated　a　reverse-volcano　relationship　with　copper　content.　Copper-rich　MMOs　(SrCu2O)　exhibited　the　highest　degradation　activity　for　MO　by　far　and　degraded　ca.　96%　MO　within　25 min.　MO　degradation　over　SrCu2O　was　a　surface-catalytic　reaction　and　fitted　pseudo-first-order　reaction　kinetics.　The　contact　between　MO　molecules　and　catalyst　surface　initiated　the　reaction　via　the　catalytic-active　phase　(Cu+/Cu2+　redox　pair),　which　serves　as　an　electron-transfer　shuttle　(⋯　Cu2　+→　+　e−Cu+→　−　e−Cu2　+⋯)　from　MO　to　dissolved　O2,　inducing　the　consecutive　generation　of　reactive　oxygen　species,　which　resulted　in　MO　degradation　as　evidenced　by　radical　trapping　experiment.　XPS　and　XRD　analysis　revealed　that　active　phases　in　SrCu2O　materials　underwent　irreversible　transformation　after　reaction,　contributing　to　the　observed　deactivation　in　the　cycling　experiment.　The　observations　in　this　study　demonstrate　the　significance　of　chemical　composition　tailoring　in　catalyst　synthesis　for　environmental　remediation　under　dark　ambient　conditions.　[Figure　not　available:　see　fulltext.].　©　2019,　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.