Institutions can contribute to regulating interethnic conflict; however, in many cases they fail to bring about lasting peace. The paper argues that their negligence of intraethnic factors accounts for some of this failure. Ethnic groups are often treated as unitary actors even though most consist of various linguistic, tribal or religious subgroups. This internal heterogeneity is often obscured by overarching collective ethnic identities that are fostered by interethnic conflict. However, when such interethnic conflict is settled, these subgroup differences may come back to the fore. This resurgence can lead to subgroup conflict about the political and economic resources provided through intergroup institutional settlements. Such conflict can in turn undermine the peace-making effect of intergroup arrangements. Different subgroup identity constellations make such destructive effects more or less likely. The paper focuses on self-government provisions in the aftermath of violent interethnic conflict and argues that lasting intergroup arrangements are especially challenging when they involve contested ethnic groups.