This study reports on a Professional Development Program (PDP) designed to raise teachers' subject specific language awareness (TSLA) as a relevant and specific element of their practical knowledge and improve their language integrated teaching behaviour. The design of the PDP was based on the interconnected model of teacher professional growth (Clarke & Hollingsworth, 2002). Data were collected using semi-structured interviews, classroom observations, and video-stimulated interviews. The PDP resulted in change in both teachers' subject-specific language awareness and related teaching behaviour. Teachers' sense of responsibility to address students' language learning appeared to be relevant for teachers' change of behaviour.