While specific MAO aluminum sites have been discussed in previous studies, a systematic classification scheme has been lacking. We developed a comprehensive classification system based on the local coordination environment of aluminum centers, with particular focus on their reactivity. The classification follows a hierarchical organization: 1) Decreasing number of Al-C bonds, with priority given to terminal over bridging methyl groups, 2) increasing number of Al-O bonds, and 3) increasing coordination number of the bound oxygen atoms. The complete classification scheme is provided in Supporting Information (Figure S-2), while Figure illustrates the most relevant site types for this study.