This study analyses the evolution of socioscientific reasoning on sustainability, of French and Australian tertiary students exchanging ideas on a digital platform, concerning local (Australian, French) environmental SSIs, and global environmental SSIs. We explore how the exchange of arguments from various disciplinary and cultural perspectives, can promote reasoning about complex problem-situations in the environment. We develop a framework of reasoning, and show how it enables a productive analysis of the nature of the exchanges, and the quality of reasoning. We argue that such a strategy may improve epistemological training on the nature of science, and citizenship