M.A. (Clinical Psychology) / Disappointments in romantic relationships can have distressing and prolonged cognitive, emotional and behavioural effects. This study explored such disappointments in the form of betrayal, rejection and the accompanying beliefs, emotions and behaviours using the Rational Emotive Behaviour Therapy theoretical framework. A theory-building positivistic case study design was implemented. Five participants completed a quantitative measure of REBT beliefs, the Shortened General Attitudes and Beliefs Scale, and participated in a semistructured interview. The experience of a romantic betrayal or rejection elicited greater irrational than rational beliefs, and more dysfunctional than functional outcomes (emotions and behaviours) for each participant. The implications for clinical practice are discussed using the REBT theoretical framework.