Despite the spread of reliable desktop audio and videoconferencing facilities, some CMC-based projects still rely on asynchronous written environments, if only because of the temporal constraints of synchronicity (Guichon, 2009; Develotte, Guichon & Vincent, 2010). Yet speaking is usually the skill students most need to improve when learning a second language (L2). This paper therefore sets out to measure the impact of distant written exchanges between Native Speakers (NS) and Non Native Speakers (NNS) on the development of NNS L2 oral output, focusing specifically on the effect of phonological nativisation. The context of this study is a teacher training programme for future primary school teachers in France. During their L2 course aiming to help them improve their mastery of English, they were given the opportunity to take part in a CMC-based project with PGCE (Postgraduate Certificate in Education) students from King's College, London. Action research was thus carried out to examine the potential of this project in the development of the participants' L2 oral output. The L2 course being task-oriented, the trainee teachers' L2 oral output was evaluated by means of pre- and post-tests based on tasks. The results show that stability prevails over progress, which is in keeping with the fact that interlanguage development is a long process (Chapelle, 2003: 119). The results also confirm the importance of phonological nativisation when learners have access only to "written" authentic input. (Contains 6 tables, 4 figures, and 1 footnote.)