Caesalpiniaspinosa (Molina) Kuntze is an extensively used plant in the industry and folk medicine, and it is characterized by containing high quantities of tannins in its pods. Ultrastructure and chemical aspects of C.spinosa leaves were analyzed to identify their properties and the function of their specialized cells. Leaflets are amphistomatic with an internal bifacial structure. Subspherical idioblasts occur on the subepidermal area of leaflets and are immersed in the parenchymatous tissue of rachis and petiole. Histochemical analyses showed that tannins are widespread on the parenchymatous tissue of leaflets, rachis, and petioles, and subspherical idioblasts present a lipophilic nature containing essential oils and lipids. Glandular trichomes occurring on the rachis and petiole bear phenolic compounds. GC–MS analysis reveals the presence predominantly of monoterpenes in the leaf essential oil. The detected compounds may be related to the antimicrobial- and antioxidant activity of C. spinosa extracts.