As the availability of adaptation finance for developing countries increases, so does the need for a transparent way of prioritising countries for the allocation of money. It is intuitive that some countries are more vulnerable to climate change than others, and that countries that are particularly vulnerable should be given priority for adaptation finance. However, research has shown that science cannot be relied upon for a single objective ranking of vulnerability. This article analyses how the Global Climate Change Alliance (GCCA), the Pilot Program for Climate Resilience (PPCR) and the Adaptation Fund currently make decisions on adaptation finance allocations. It finds that each of the funds uses vulnerability to prioritise among countries, but the criteria applied vary and other criteria also play a role. Thus, vulnerability is politically, as well as scientifically, ambiguous. The Cancun Agreements have not resolved this, leaving a challenge for the Green Climate Fund.