While peace processes increase the likelihood that a civil war is resolved, they can also complicate peace by increasing the risk of rebel fragmentation. In this article, we argue that negotiations exacerbate pre-existing structural and substantial divisions within rebel organizations, therefore increasing the likelihood of a rebel split. More specifically, we put forward a theoretical framework that specifies why factions within a rebel group may disagree with the onset of negotiations, the conclusion of a peace agreement, or the implementation of an agreement—and thus break away during the peace process. We empirically assess the merit of this framework by systematically comparing the impact of these phases in a peace process on the fragmentation of rebel organizations. Using data that more accurately reflect the moment a rebel split takes place than earlier studies, we find that peace processes have a greater substantial impact on rebel fragmentation than previously assumed.