Betrayal is the sense of being harmed by the intentional actions or omissions of a trusted person. The most common forms of betrayal are harmful disclosures of confidential information, disloyalty, infidelity, dishonesty. They can be traumatic and cause considerable distress. The effects of betrayal include shock, loss and grief, morbid pre-occupation, damaged self-esteem, self-doubting, anger. Not infrequently they produce life-altering changes. The effects of a catastrophic betrayal are most relevant for anxiety disorders, and OC D and PTSD in particular. Betrayal can cause mental contamination, and the betrayer commonly becomes a source of contamination. In a series of experiments it was demonstrated that feelings of mental contamination can be aroused by imagining unacceptable non-consensual acts. The magnitude of the mental contamination was boosted by the introduction of betrayal themes. Feelings of mental contamination can also be aroused in some ‘perpetrators’ of non-consensual acts involving betrayal. The psychological significance of acts of betrayal is discussed.