This article aims to contribute to the knowledge of how the 'scaling' of Education for Sustainable Development (ESD) activities is conceptualized in practice through transactional learning encounters. In the context of the UNESCO Global Action Programme (GAP) on ESD, I discuss the re-actualisation of experiences as part of these encounters. The study is a result of data collected as part of a Re-Solve participatory research workshop held in South Africa in 2016, involving researchers and practitioners with experiences of ESD-activities in the Southern African region. To identify and analyse the transactional learning encounters a practical epistemology analysis (PEA) is used. The article draws on a Deweyan theory of learning as transactional encounters, supported with a tentative conceptual framework of scaling-ESD-activities-as-learning (SEAL). Throughout the study, I illustrate the transactional encounters, including the re-actualisation of participants' past experiences of ESD-activities. These encounters enabled the conceptualization of contextually relevant concepts of scaling, thus constituting an enabling condition for reflective scaling practices.