Cu-exchanged zeolites are widely used redox catalysts and the oxidation state of Cu is crucial in understanding their performance over a wide range of applications. Interestingly, a fraction of Cu sites in zeolites is reported to reduce at high temperatures in the absence of a reducing agent and as of today a detailed understanding of this process is still missing. In this contribution, we use first principles-based phase diagrams to explore thermodynamic driving forces for the autoreduction of Cu sites in the zeolite SSZ-13. We find that mainly monovalent Cu(II)–OH sites anchored at well-separated Al atoms and to a lesser degree di-hydroxyl Cu dimers drive the autoreduction of Cu sites in the zeolite SSZ-13. Using these insights, we can reproduce experimental trends in autoreduction reported in the literature. This work gives detailed insights into the autoreduction of Cu sites in SSZ-13 and demonstrates that the nature of Cu sites in the zeolite SSZ-13 depends on the exact conditions the material is exposed to. Optimizing these reaction conditions might allow to improve the performance of Cu-exchanged zeolites over a wide range of applications.