We study the effects of carbon dioxide taxation on fossil-fuel markets where the supply is regulated by a resource-exporting cartel. To clarify some previous results, we show that the optimal tax set by a coalition of buyers is not a neutral Pigouvian tax but a combination of a tariff and Pigouvian tax. Because of the tariff element, the tax can shift more rents from the cartel than the pollution causes damage-related costs. Thus, the pollution problem accompanied by the coordination of carbon taxation can entail net benefits for the buyer side.