Rational Emotive Behaviour Therapy (REBT) hypothesizes that the functionalityof inferences is primarily affected by the preferential and demanding natureof rational and irrational beliefs, respectively. It is then, secondarily,influenced by the functional and dysfunctional contents to which rationaland irrational beliefs, respectively, refer. This hypothesis was tested byasking 96 participants to imagine themselves holding one of four specificbeliefs: a rational belief with a preference and a functional content, anirrational belief with a demand and a dysfunctional content, a rationalbelief with a functional content and no preference, and an irrational beliefwith a dysfunctional content and no demand. Participants imagined themselvesholding their belief in an imaginary context, whilst rating the extent oftheir agreement to 14 functional and dysfunctional inferences. Contrary toREBT theory, results indicated that rational and irrational beliefs had thesame magnitude of effect on the functionality of inferences, whether theyreferred to a preference/demand+contents, or only contents. The discussionmaintains that preferences and demands may not constitute the principalmechanism through which rational and irrational beliefs affect thefunctionality of inferences. Instead, consistent with Beck'scognitive therapy, belief contents may constitute this primarymechanism.