Scientific expertise and outcomes often give rise to controversy. An educational response that equips students to take part in socioscientific discussions is the teaching of Socially Acute Questions (SAQs). Students engaging with SAQs need to engage with socio scientific reasoning, which involves reasoning with evidence from a variety of fields other than science, including values, economics, local and global perspectives, governance issues, and a variety of stakeholder perspectives. This paper describes research in which tertiary students from France and Australia engaging with sustainability SAQs reasoned responses in wikis. In previous analyses (Authors, 2013), we have developed a six-dimensional framework of socio scientific sustainability reasoning (S3R). In this paper, we describe the development of an interactional reasoning framework that captures the quality of reasoning in the online discussions that led to specific improvement in S3R. The framework draws on and extends the discourse framework of Mercer, and Habermas' three worlds of validity of arguments to identify first that a particular category--integrated exploratory talk--is associated with improvement in S3R, and that this almost exclusively involves exchanges drawing on personal and social evidence claims as well as technical. We explore the implications for supporting collaborative decision making in socio scientific issues, for democratic participation and for post normal science education.