Cities, and particularly urban local governments, are now widely recognised for their part in the complex, multilevel landscape of climate governance and carbon reduction. Nonetheless local government projects and initiatives are often framed as of limited value, outside the formal governance framework, and unable to contribute systematically. In contrast, this paper locates these initiatives as already part of the way in which governing climate and carbon is conducted and as governance experiments. We provide a descriptive analysis of these initiatives across Australia's capital cities, highlighting the domains, mechanisms, and partners through which they operate. We illustrate the enactment of experimentation through a detailed examination of the Sydney-based initiative termed Treading Lightly, drawing out in particular the workings of institutional experimentation and experimentation in governance practices. We conclude with brief reflections on the governance implications of such experimentation and their importance as a site in the emergent politics of urban carbon governance.