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The synthesis of benzoxazines is a fundamental step in obtaining polybenzoxazine resins, understanding its formation mechanism is important to improve the properties of these resins. This paper analyses the Mannich-type reaction between o-dianisidine, bisphenol A (BPA) and formaldehyde. Spectroscopic studies and computational calculations revealed that this reaction only produced a monobenzoxazine, resulting from the formation of cyclic arrays by intermolecular hydrogen bonds between o-dianisidine and BPA. Such cyclic arrays prevented bifunctional benzoxazine and/or oligomer formation.