Knowledge of chemical composition is fundamental to human nutrition, contributing to the quality of foods. Phenolic compound in both fruits were analyzed by HPLC-DAD and the total flavonoid content was determined by the Folin-Ciocalteu assay. The antioxidant activity was evaluated by the total reactive antioxidant (TRAP) method. Psidium cattleianum presented a higher content of polyphenolic compounds than P. cattleianum var. lucidum (501.33 and 292.03 mg/100 g, respectively), with hyperoside being one of the major flavonoids identified for both cultivars. In addition to flavonoids, P. cattleianum presented an anthocyanin, identified as cyaniding. The antioxidant activity varied in a concentration-dependent manner for both strawberry guava species. The volatile oils in fruits and fatty acids in seeds were quantified by GC-EM. The analysis of the essential oil of yellow strawberry guava was compared with a previous study quite similar and was especially characterized by the presence of unsaturated fatty acids (86.25% and 76%, respectively), among which linoleic acid as the most abundant.