Territorial self-governance remains a persistent feature of contemporary conflict resolution. Existing research has identified a number of exogeneous factors that impact the sustainability of such arrangements, including previous levels of violence, the level of economic development in a given territory, or the strategic importance thereof. We argue that a hitherto neglected variable lies in the form of the autonomy agreement, namely in its degree of legalization. Based on an in-depth qualitative evaluation, we assess the degree of legalization of 228 post-conflict autonomy agreements signed between 1945 and 2020. Quantitative survival analyses and Cox regression models show that a higher degree of legalization has a positive and significant effect on peace durability.