Increasingly, teacher clusters are being used as a substitute for the more traditional approaches to the professional development of teachers. With this goal in mind, many provincial education departments in South Africa have sought to institutionalise and encourage the formation of teacher clusters as vehicles for the continuing professional development of teachers. What are the challenges of this institutionalisation and to what extent has it served teachers in their quest to learn from and with each other in clusters? In this article, we use a qualitative case study approach to examine the dilemmas of the institutionalisation of teacher clusters in the Mpumalanga province of South Africa. Using mostly interview data with all the key central office administrators responsible for science and mathematics in the province, and a sample of the participating cluster (teacher) leaders and observations of their cluster activities, we discuss how the institutionalisation processes may have led to rather undesirable outcomes. We examine the way in which institutionalisation may have resulted in a reduction of the "opportunities to learn" for the participating teachers. We argue that, while the intentions of the policymakers to provide support and recognition for the teacher clusters were noble and progressive, the consequences of their intervention were somewhat negative and tended to bureaucratise clusters, thereby alienating teachers from these traditionally bottom-up structures of professional development. We conclude the article by exploring what the possibilities are for teachers reclaiming the spaces created by the teacher cluster "movement" in South Africa. (Contains 2 endnotes.)