Rational Emotive Behaviour Therapy (REBT) hypothesizes that the functionality of inferences is primarily affected by the preferential and demanding nature of rational and irrational beliefs, respectively. It is then, secondarily, influenced by the functional and dysfunctional contents to which rational and irrational beliefs, respectively, refer. This hypothesis was tested by asking 96 participants to imagine themselves holding one of four specific beliefs: a rational belief with a preference and a functional content, an irrational belief with a demand and a dysfunctional content, a rational belief with a functional content and no preference, and an irrational belief with a dysfunctional content and no demand. Participants imagined themselves holding their belief in an imaginary context, whilst rating the extent of their agreement to 14 functional and dysfunctional inferences. Contrary to REBT theory, results indicated that rational and irrational beliefs had the same magnitude of effect on the functionality of inferences, whether they referred to a preference/demand+contents, or only contents. The discussion maintains that preferences and demands may not constitute the principal mechanism through which rational and irrational beliefs affect the functionality of inferences. Instead, consistent with Beck's cognitive therapy, belief contents may constitute this primary mechanism.