Introduction. Many underutilized wild fruits have great nutritional and functional potential, providing chemical compounds with biological properties. Materials and methods. In the present work we quantified bioactive compounds such as vitamin C (ascorbic and dehydroascorbic acids), and total phenolic compounds composed mainly of phenolic acids, flavonols and anthocyanins, as well as evaluating the antioxidant capacity through different in vitro tests (Folin-Ciocalteu, ABTS·+, DPPH· and FRAP) in wild blackthorn (P. spinosa L.) and hawthorn (C. monogyna Jacq.) fruits of Spanish origin, including samples from different seasons and locations. Results and discussion. As expected, wide variability was found in the composition of fruits of the same species, which justifies the necessity of analyzing several batches of wild fruits, in order to have representative results taking into account the natural variability. Fruits of P. spinosa showed vitamin C content ranging between (5.14 and 15.35) mg·100 g–1 fw (mainly dehydroascorbic acid); total phenolic compounds ranged from (1851 to 3825) mg·100 g–1 fw, characterized by a high content of anthocyanins and phenolic acids. Fruits of C. monogyna presented (16 to 39) mg vitamin C·100 g–1 fw and (449 to 1438) mg total phenolic compounds·100 g–1 fw, characterized by a high content of phenolic acids and flavonols. Antioxidant capacity was higher for P. spinosa fruits than for C. monogyna fruits; DPPH· values showed a strong correlation with vitamin C, while phenolic compounds were a major contributor to the antioxidant activity of these fruit extracts. Fruits of P. spinosa and C. monogyna should be reconsidered as new valuable sources of safe and inexpensive antioxidants.