This article asks whether the Covenant of Mayors – a transnational network that aims to reduce CO2 emissions in European towns and cities – amounts to transnational environmental regulation. It answers in the affirmative. The relevance of treating the Covenant of Mayors as transnational environmental regulation is that this triggers qualitative expectations about how decision-making power will be wielded within the network. The article indicates that the Covenant is likely to clear some of these qualitative hurdles, but may struggle with others. This calls for further deliberation on how to bolster the legitimacy of transnational environmental regulation without forsaking its flexibility and innovative drive. Considering that the outlook for the swift, successful adoption of a global, binding regulatory regime for climate change is bleak, the world will likely remain dependent on alternative, transnational initiatives as drivers for change. The ramifications of this debate therefore considerably exceed the scope of the Covenant of Mayors itself.