Résumé : This dissertation investigates language policy in a Dutch-medium secondary school in Brussels. The school in question endeavours to implement a language policy in which languages other than Dutch are formally included, which is in stark contrast to their peers, who often implement a strict, Dutch-only policy in order to respond to the increasing linguistic diversity and “Frenchification” in Brussels Dutch-medium education. This thesis addresses the question of how the teachers negotiated such a pro-multilingual language policy in this setting. The research is designed as a (socio)linguistic ethnographic case study based on a conceptualisation of language policy as (1) operating on different levels; (2) consisting of three components; and inseparable from the social world in which it is effectuated. The study combines ethnographic field work and participant observation with interview data, linguistic analyses of interactional data, document analysis and analyses of elements of the linguistic landscape to gain insights into the nature and extent of the school’s unique pro-multilingual project. Although the school profiles itself as an institution which aims to prepare its pupils for future educational and professional success by increasing their language skills, the school’s policy declarations harbour an ambivalent stance vis à vis multilingualism. In terms of individual teachers’ perceptions and practices, then, we demonstrate that they, too, voiced contradictory sentiments and displayed behaviour in the classroom which was at once welcoming of pupils’ use of linguistic resources other than monolingual Dutch, and restrictive of it.