Little research exists on how therapists facilitating sex offender treatment programmes in prisons view the change process in their clients and its impact upon themselves as mediators of change. This study explored therapists' perceptions of this, the challenges situated within this role and the wider social context. An opportunity sample of four sex offender treatment therapists, averaging 6 years' experience, engaged in semi-structured interviews exploring their perceptions as mediators of change. Transcripts were subject to Foucaultian discourse analysis. Four central interpretative narratives emerged: the sex offender identity; the therapist role; the impact of the situational context; and the perceived constraints encountered within the change process. A fundamental dissonance emerged between the aims of the therapeutic process and those of a custodial environment, constraining change in offenders. It is argued that “what works” guidelines, while promoting a systematic approach, may have also created a mono-therapeutic culture, possibly hindering effective programme outcomes.