Within the English language teaching profession there is now general acceptance of the advantages of integrating language and content teaching. Yet challenges continue over how such integration can be accomplished within individual classrooms. In Asian universities using English as the medium of instruction (EMI), one of the key barriers to integration is the alleged reticence of learners to participate in spoken discourse within the classroom. If the language policy aims of these institutions are to be realized, these constraints need to be acknowledged. This paper uses a multidimensional investment framework to investigate the oral participation of one group of foreign language learners within their English for academic purposes (EAP) classroom at an EMI university in Hong Kong. Drawing on data from interviews, classroom observations, audiovisual recordings of classroom interaction, and documentary analysis, this paper argues that the successful integration of language and content teaching should include an appreciation of the institutional forces that constrain and enable learners' oral investment, how learners deploy a variety of knowledge, skills and understandings in support of this investment, and the degree of freedom learners enjoy in shaping the processes and products of their investments. The implications of this research for classroom practices are explored. Copyright © 2008 John Benjamins Publishing Company.