Following the success of the High Ambition Coalition (HAC) in contributing to the UNFCCC Paris Agreement in 2015, the European Union (EU), which played an instrumental role in the coalition, remobilised, along with its partners, the HAC to support two key international climate agreements in 2016: the Kigali Amendment to the Montreal Protocol and the ICAO Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA). Despite these negotiations taking place simultaneously within a push for climate action following COP21 and having significant EU involvement, the HAC produced mixed results. While the HAC appeared successful in helping to secure an ambitious agreement in Kigali, thanks to broad involvement from the EU and HAC partners, this was not the case with ICAO CORSIA, where the EU struggled to mobilise its HAC partners. This article answers the question Why was the EU successful in its involvement with the High Ambition Coalition in the negotiations leading to the Kigali Amendment yet unsuccessful in the ICAO CORSIA negotiations? In conducting a detailed “most similar” case study comparison, it identifies three contextual (scope) conditions that needed to be present for such success: capacity of the HAC for collective action, a favourable institutional environment in the negotiating forum in question, and the availability of sufficient time to influence the negotiations.