Research has identified several factors that impact the sustainability of post-conflict territorial autonomy arrangements (TAA), including previous levels of violence, economic development in a given territory, or the strategic importance thereof. We argue that a hitherto neglected variable lies in the legal form of the autonomy agreement – that is, the degree to which it has been ‘legalized’ by the language and processes prescribed in the agreement. Based on a qualitative evaluation, we assess the legalization degree of 236 TAA signed between 1990 and 2019. Survival analyses and Cox regression models show that a higher degree of legalization has a positive and significant effect on peace durability.