In this paper we describe Expanding Textual Repertoires (ETR) as a framework for enhancing Singaporean secondary school teachers’ understanding of the nature and effective teaching of the increasingly specialized language and literacy competencies required of their students for skilled participation in the curriculum areas at the secondary grade levels. Drawing from a larger intervention project involving the development, and the introduction to teachers, of the three principles of ETR (intellectual quality, connected learnings, explicit instruction), we examine in what ways it could bring greater coherence, keener awareness of the language and literacy demands, as well as a more confident and critical engagement with the subject knowledge to the work and classroom interactions of one of the secondary Social Studies teachers. Looking at her post-intervention delivery of a unit of work, we show how the Social Studies teacher developed and trialled new teaching materials and tasks in the light of her broadened understandings of effective teaching through the ETR framework, refining and modifying them by ongoing analysis of the classroom learning environment and her students’ responses. Through the analysis of ongoing sequences of lessons across a unit of work, we document the ‘developmental history’ [Christie, F. (2002). Classroom discourse: A functional perspective. London: Continuum] of her classroom, showing how it unfolded over time as a text, how the teacher handled different stages of learning, if and how students’ prior learning and everyday knowledge were built on, if and how connections between various texts and literacy activities were made across lessons, how new learning, appropriate technical language and patterns of reasoning were accumulated as the unit progressed, and what evidence there was of students’ uptake of this new learning through literate talk in their lesson contributions.