This study tested the rational-emotive-behaviour theory hypothesis that, when imagining having a serious disagreement with one's romantic partner or closest friend, imagining believing the rational belief that disagreement is not destructive will lead to less dissatisfaction with either relationship than imagining endorsing the irrational belief that disagreement is destructive. The influence of demand characteristics on such an effect was assessed with two counter-demand control conditions. Undergraduates (106 women and 44 men) were assigned randomly to one of five conditions in a pretest/posttest design. The belief appeared to be effectively manipulated. Post-test relationship satisfaction was significantly higher in the rational condition than in the control or irrational condition. These results were not affected by the counter-demand instructions and suggest that this rational belief may reduce relationship dissatisfaction during disagreements.