Today phenols are increasingly considered to be the most useful phytochemicals in the food due to the extremely good antioxidant activity. Positive antioxidant effects of certain phenolic compounds are well explored and the differences in their activity were ascribed to the differences in their chemical structures. The aim of this study was to determine the antioxidant activity of the selected phenolic acids from the subgroup of cinnamic acid derivatives (p-coumariç, ferulic, synapic, caffeic and rosemarinic acid), as well as their equimolar mixtures with two or more acids, using the method of determining the reduction power of the sample (Ferric Reducing / Antioxidant Power, FRAP). A possible interaction (synergistic / additive / antagonistic) effects between the tested phenolic acids were also investigated. The results of this study confirm the differences in antioxidant activity between the tested phenolic acids as well as their equimolar mixtures. Based on the obtained results, it can be concluded that the caffeic acid along with its esther rosmarinic acid showed the best antioxidant activity among the selected cinnamic acid derivatives tested while p-coumaric acid showed the weakest activity. The best and strongest synergistic effect was confirmed at the lowest concentrations of the mixtures tested, while the highest number of mixtures showed a synergistic effect at the highest concentrations tested.