Since the 2010s, civil society initiatives have been developing in the fields of the environment and the social economy. They constitute a heterogeneous group of collectives tackling socio-environmental issues which, in the view of their promoters, are insufficiently addressed by public authorities. The actions undertaken are diverse: improving the living environment, protecting the environment, healthy eating, recycling, reusing, sustainable mobility, renewable energy. Over and above the specific features of their activities, the aim is to understand the type of socio-ecological transformation to which these mobilizations are contributing. These collectives usually start from observations of the malfunctioning of existing political and economic systems, both in terms of their impact on the environment and on health, as well as the loss of social ties and the weakening of collective capacity to act. They then form "publics" (in Dewey’s sense) to intervene in the regulation of problems, offering local spaces and means of resistance to the dominant socio-technical, political and economic regimes. Their approach to transition issues is transversal, in the way they integrate social and environmental issues and raise the question of change from the perspective of everyday practices. In this article, we analyze the contributions of these collectives to socio-environmental transformations from two complementary angles. The first is that of experimentation, based on the idea that citizens are endowed with agency and a capacity to test concrete solutions to problems that concern them. The second concerns the relations that these collectives maintain with local public authorities, and their contribution to a more or less advanced democratization of climate issues. To do this, we draw on the contrasting cases of two Greater Paris communes, Clamart and Ivry-sur-Seine. The comparison allows us to analyse the influence of local contexts and socio-political trajectories on the potential for local social-ecological transformation.