Computer-mediated communication (CMC) is currently used in language teaching as a bridge for the development of written and spoken skills [Kern, R., 1995. "Restructuring classroom interaction with networked computers: effects on quantity and characteristics of language production." "The Modern Language Journal" 79, 457-476]. Within CMC technologies, one of the applications in language learning involves student participation in online discussion forums (DFs), which can be conductive to enhanced learning outcomes. The purpose of this paper is to analyze modal verbs, a characteristic of the language produced in asynchronous DFs and to confirm whether the conversational elements present in the written communication mode reflect the oral traits attributed to synchronous interactions. Our research is based on the analysis of a corpus of international forum interactions built up over several years. We compare the results of our analysis with those found in two other studies: Yates' examination [Yates, S.J., 1996. "Oral and written linguistic aspects of computer conferencing: a corpus-based study." In: Herring, S. (Ed.), CMC: "Linguistics, Social and Cross-Cultural Perspectives." John Benjamin, Amsterdam, pp. 29-46] of modal auxiliary use in a corpus containing CMC, speech and formal writing, and Pique et al.'s study [Pique, J., Posteguillo, S., Andreu, J.V., 2002. "Epistemic and deontic modality: a linguistic indicator of disciplinary variation in academic English." "LSP and Professional Communication" 2 (2), 49-65] of research articles as samples of written scientific discourse. Our analysis highlights the differences in the way that modal auxiliaries are used in text-based CMC. From our study, we can infer and understand some of the potential offered by computer-mediated learning tools which, in turn, will contribute to the knowledge required in the design and production of technology-mediated educational activities. (Contains 4 tables.)