We assess the development of informality in international climate policy on two levels: Whether informal organizations meaningfully contribute to climate change mitigation, and what role informality plays under the UN Framework Convention on Climate Change (UNFCCC). Proliferation of informal intergovernmental organizations (IIGOs) has enabled the move from a rigid list of countries with and without commitments, to the Paris Agreement, under which each country contributes to mitigation. Within the UNFCCC process, we find a “formality-informality cycle,” in which actors sometimes render rules and procedures more flexible and hence more efficient, only to suddenly reverse this trend at other times. Such a high-profile reversal occurred in Copenhagen in 2009. Subsequently, through the use of highly transparent negotiation procedures, trust in informality increased again, allowing negotiators to successfully override Nicaragua’s opposition in Paris in 2015. Similar formality-informality cycles can be observed on specific topics within the UNFCCC negotiations, such as international market mechanisms.