This presentation explores the dialogue generated in two asynchronous online forum discussions when students and staff in an educational psychology subject were provided with an interaction guide to facilitate collaborative knowledge construction. The analysis is informed by the dual perspectives of systemic functional linguistic theory (Halliday 1978) and sociocultural theory (Vygotsky 1978, Daniels 2010). Combining these frameworks allowed us to explore the dialogue from both the broad contextual or macro-level perspective of contemporary knowledge practices in tertiary settings and the unfolding or micro-level language choices as participants interact move by move to construct the learning environment. Because SFL theory approaches cognition from the perspective of meaning-making, the linguistic data provide useful insights into students’ learning and the co-construction of knowledge (in other words, their changes in meaning making) (Halliday & Matthiessen 1999). Our major interest here lies with tracing the ways in which core concepts or ideas from educational psychology are introduced into discussion, how they are negotiated among participants, and the nature of common understandings arrived at. Preliminary results showed the guides fostered a high level of involvement of all the students in peer interaction and an apparent consciousness about the relationship between interpersonal and knowledge-based contributions. Drawing on systemic functional linguistics (after Halliday & Matthiessen 2004; Martin & Rose 2007), the analysis reveals the fluid nature of ideas, the relations between academic concepts and students’ everyday and professional experience, and the role of carefully orchestrated language choices in creating intersubjectivity. In this way, we gain insights into ‘the sequential and contingent development of concepts over time’ (Daniels, 2010); in other words, in what Maton (2014) has described the ‘wave-like’ nature of the knowledge building. The contribution of Maton’s work in the sociology of knowledge enables us to profile the unpacking of theoretical concepts into accessible forms of knowledge and their repackaging into more abstract ideas through the forum contributions by participants. Thus the importance of the interpersonal as the gateway to the experiential (Halliday 1993) is evident. The findings suggest a unique place for the asynchronous online discussion forum in enabling the interplay of guided reflection, sociality and individual reflexivity. It also confirms the importance of the expert as mediator in the process of collaborative knowledge construction.Daniels, H. (2010), ‘The mutual shaping of human action and institutional settings: a study of the transformation of children's services and professional work’. British Journal of Sociology of Education. 31:4, 377-393.Halliday, M.A.K. (1993). Towards a Language Based Theory of Learning. Linguistics in Education. 5, 93-116.Halliday, M.A.K. (1978). Language as a Social Semiotic. London: Edward Arnold.Halliday, M. A. K. and Matthiessen, C. (2004). An Introduction to Functional Grammar, London, Hodder Headline Group.Halliday, M. A. K. and Matthiessen, C. (1999). Construing Experience through Meaning: A Language –based Approach to Cognition. (Open Linguistics Series). London: Continuum. Martin, J. R. and Rose, D. (2007). Working with Discourse: Meaning Beyond the Clause, London, Continuum.Maton, K. (2014). Knowledge and Knowers: Towards a realist sociology of education. Abington & New York: Routledge.