Analyze the psychometric properties of a new measure for the assessment of the irrational beliefs proposed by Ellis in his model of Rational-Emotive-Behavioral Therapy (REBT): the Scale of Irrational Contents and Styles-Basics (SICS-B). The SICS-B evaluates the “contents” of the three basic irrational beliefs (Ap: Approval, Pe: Perfectionism, and Co: Comfort) in relation to the four different “styles” (inferences or processes of thought) in which they can be expressed (DEM: Demandingness, AWF: Awfulizing, FI: Frustration Intolerance, and CON: Condemnation). 259 respondents (79.5% women) undergraduates (63%) and (post)graduates (37%) between 18–63 years (M = 26.5, SD = 10.7). A set of 72 statements were drafted (half in a rational way) using a 5-point Likert scale to cover the three areas of content and the four styles of irrational thinking (3 × 4 subscales), of which to choose the three items of each area with highest psychometric indices and elaborate with them the definitive scale of 36 items. The reliability and validity study was carried out through a comparative item analysis (average inter-item correlations, item-total correlations) and internal consistency (Ordinal α) of each subscale, and confirmatory factor analyses of several models. The SICS-B presents good indices of internal consistency and content validity. Confirmatory factor analyses validate the 3 × 4 model in which the three specific contents of the irrational beliefs are integrated with the four main styles of irrational thinking as proposed by the REBT. The SICS-B is a reliable and valid short scale for the assessment of irrational beliefs.