Abstract Pterospartum tridentatum (L.) Willk., Gomphrena globosa L. and Cymbopogon citratus (DC) Stapf. are examples of medicinal plants that demand a more detailed characterization. Therefore, phenolic composition (e.g., phenolic acids and flavonoids) was analyzed by chromatographic and mass spectrometry techniques and the antioxidant activity was also accessed through free radicals scavenging activity, reducing power and inhibition of lipid peroxidation in brain homogenates. C. citratus revealed the highest β-carotene bleaching and lipid peroxidation inhibitions, being luteolin 2″-O-rhamnosyl-6-C-glucoside the main compound. P. tridentatum presented the highest 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity and reducing power and mainly dihydroflavonol and isoflavone derivatives were detected. Otherwise, G. globosa presented kaempferol 3-O-rutinoside as the most abundant phenolic compound and betacyanins were only present in this sample. It is very interesting to study the phytochemical composition of these plants, given the importance of their consumption.