Background: Mental contamination is a phenomenon whereby people experience feelings of contamination from a non-physical contaminant. Rachman (2006) proposes that standard cognitive behavioural treatments (CBT) need to be adapted here and there is a developing empirical grounding supporting the concept, although suggestions on adapting treatment have yet to be tested. Method: A single case study is presented of a man with a 20-year history of severe treatment resistant Obsessive Compulsive Disorder (OCD) characterized by mental contamination following the experience of “betrayal”. He was offered two consecutive treatments: standard CBT and then (following disengagement with this) a cognitive therapy variant adapted for mental contamination. Clinician and patient rated OCD severity was measured at baseline and the start and end of both interventions. Results: Six sessions of high quality CBT were initially attended before refusal to engage with further sessions. There were no changes in OCD severity ratings across these sessions. A second course of cognitive therapy adapted for mental contamination was then offered and all 14 sessions and follow-ups were attended. OCD severity fell from the severe to non-clinical range across these sessions. Conclusions: The need to consider adapting standard treatments for mental contamination is suggested. Limitations and implications are discussed.