This article looks at the development of mathematisation in a primary school mathematics teacher's classroom. The teacher was a volunteer in a professional development programme that promoted didactisation principles for mathematics teacher development. At the heart of didactisation lies the principle of mathematisation. Mathematisation as a fundamental aspect of teaching and learning in mathematics classrooms led us to seek ways to develop the potential for mathematisation in teachers. This article explores how a grade 5 mathematics teacher improved mathematisation in her classroom while she participated in a didactisation-based professional development intervention. Over a period of a year, four lessons were audio-recorded and transcribed. Design research constituted the framework of the professional development programme, while model-eliciting tasks were used as a vehicle for programme delivery. The Realistic Mathematics Education principles of guided reinvention, mathematisation and contextual exploration provided the theoretical framework for analysis and coding of the data. In addition, Gravemeijer's framework of emergent modelling and Douady's conceptualisation of tool and object use in mathematics provided the basis for lesson analysis at micro-level. The results show that this teacher was able to change the quality and level of mathematisation in her classroom through improving student mathematical activity, which led to redefining her role in the classroom. Her guiding of the learning process moved from telling to asking questions and, finally, to facilitating mathematical re-invention.