Common Article 3 to the four Geneva Conventions encourages the parties to a noninternational armed conflict to bring into force international humanitarian law provisions through the conclusion of special agreements. Since armed groups are ever more frequent participants in contemporary armed conflicts, the relevance of those agreements as means to enhance compliance with IHL has grown as well. The decision-making process of special agreements recognizes that all the parties to the conflict participate in the clarification and expansion of the applicable rights and obligations in a way that is consistent with the principle of equality of belligerents. This provides incentives for armed groups to respect the IHL rules they have themselves negotiated. However, even upon the conclusion of such agreements, it remains unclear which legal regime governs them. This paper will argue that special agreements are governed by international law instead of domestic law or a sui generis legal regime. Keywords: armed groups, special agreements, sources of international law, equality of belligerents, participants.