In 2001 the Gas Exporting Countries Forum (GECF) was created by some of the world’s leading natural gas producing and exporting countries in order to promote their mutual interests through cooperation, in particular with regard to extracting the maximum value from their natural gas exports. My core research question is: Does cooperation among GECF member countries explain those exporters’ market power in highly import-dependent natural gas consuming countries? To determine the influence of the GECF’s cooperative actions and policies, I study the GECF’s cooperative behaviour and measure the role of (collusive) producer conduct in terms of its contribution to achieving the main GECF objective: attaining gas prices that are measurably above the cost of production and hence help producers earn significant economic rents. I employ a variety of methods from the international relations literature on cooperation and cartelisation, collective action theory and an economic measurement model in three case studies. I find that cooperation among GECF members partly explains their market power in a number of import-dependent gas markets. This is so despite the GECF’s weak degree of institutionalisation. The reasons for the GECF’s influence on effective cooperative results are: first, conducive structural conditions in many gas importing markets favouring cartelisation; second, GECF members use methods such as artificial market entry barriers (e.g. long-term term contracts negotiated in a non-transparent way) to secure their market power and third, the GECF faces less severe internal procedural challenges that plague other cartels such as collective action problems, especially cheating. Cooperation among GECF exporters hence contributes to high(er) prices of natural gas. This causes economic inefficiencies and a transfer of wealth—and political power—from gas consumers to producers. It also hinders climate change mitigation as cleanerburning gas remains too expensive to replace ‘dirty’ coal in power generation.