--- - |2 Abstract Case formulation is considered important in both the development of the therapeutic relationship and in starting the process of therapeutic change. Cognitive analytic therapy describes the developmental origins and maintenance of a client's problems in both written (reformulation letter) and diagrammatic form (Sequential diagrammatic reformulation; SDR). This study aimed to investigate the effects of these reformulation tools on insight and symptom change. A small‐N repeated measures design was employed with quantitative and qualitative measures collected from six therapist/client dyads. Participating therapists kept a record of their delivery of CAT reformulation tools. Participating clients completed the insight sub‐scale of the Self‐Reflection and Insight Scale (SRIS‐IN) every fourth session and the Clinical Outcomes in Routine Evaluation‐10 (CORE‐10) every session. Qualitative data from client change interviews regarding their experiences of CAT and attributions of change was explored using template analysis. Participants demonstrated improvements (symptom reduction and insight increases) over the course of the intervention. Administration of reformulation tools did not consistently result in significant changes on insight and symptom measures. However, the tools were identified by participants as leading to insight and emotional change within the context of a good therapeutic relationship. These findings suggest that a genuine therapeutic relationship is an important change mechanism operating through, and strengthened by, CAT‐specific tools. - Clinical Psychology & Psychotherapy, Volume 0, Issue ja, -Not available-.