Résumé : Working with mathematics curriculum materials to create learning opportunities for pupils, and at the same time to develop mathematical knowledge for teaching is an important part of the work of teaching. In this study I investigate the role of curriculum materials, more precisely a task analysis schedule, as catalyst for teacher learning. I explore the different forms of feedback resulting from developing and working with a' tool' designed to analyse mathematical tasks/curriculum materials for instruction. The results are based on the work with two lower secondary school teachers in England, John and Paul, as they work together, over the period of nine months, in a group of teachers and with a university academic, on the analysis, development and teaching of mathematics curriculum materials. It is argued that the task analysis 'tool' develops, to become a 'catalytic tool' to provide feedback for teacher learning. In the process it affords feedback loops and changes its character, from 'tool' as artefact to 'epistemic object' at the interface between task design and enactment. The results provide deeper insights into the processes of teacher learning with the help of analytic tools and the feedback these may afford.