Large-scale socio-environmental education to foster a culture of sustainability and transform degrading situations is considered a matter of worldwide urgency. Considering this, we present and discuss results from a thesis that investigated the potentials and challenges of a participatory methodology, capillarity architecture, to educate sustainability leaders used on a socio-environmental training programme for public servants from a Brazilian Higher Education Institution. Capillarity architecture, launched by the Brazilian Federal Government in 2005, is a methodological strategy that encompasses participatory action research and collective networks of educators for training in an exponential format. The participants in the process undergo training while developing educational practices for their university peers. The overall study included 2500 individuals, who were assigned to one of 160 learning collectives in seven university campuses. The research method includes a participatory-research approach, a focus group, participant observation and discursive textual analysis of process documents. The results are discussed considering two points: (1) Impacts of capillarity architecture on the university experience and; (2) Challenges and tensions between the commitment to leadership formation on a large scale and the search for emancipation in EE. It can be concluded that even considering the permanent risk of impairing quality in exponential Environmental Education training, capillarity (in its critical principles) allows for: gains in autonomy, critical reflection, exchanging experiences and repertoires within the university, educating new leaders, in addition to the concomitant engagement of a larger audience. The results of this project also provide us with evidence that leadership formation in sustainability is influenced by the access or not of the participants and coordinators to material conditions (budget, infrastructure, personnel). The relevance of promoting concrete investments in environmental education and training towards sustainability is indicated, so that large-scale training can take place in territories that have significant social inequalities. Capillarity architecture questions the traditional format of organisational training. It provides for the exchange of experiences among its participants, elaboration and development of courses in the socio-environmental area in a collective way, based on problems and social relations of the local territory. These are fundamental aspects when we think about leadership formation on a large scale in broad policies, which involve different cultures and diverse socioeconomic conditions worldwide.