In this study, 11 hydroxybenzoic acids, 6 hydroxycinnamic acids, 6 flavonoids, and 2 synthetic phenolic antioxidants were evaluated according to their scavenging capacity and structure relationships. The IC50 was calculated for all compounds and the effects of the concentration of antioxidant and the length of the reaction on antioxidant capacity were taken into consideration. Based on the data of tested phenolics some structure-activity relationships were suggested and discussed in detail. Poor correspondence of the results between ABTS+• and DPPH• assays was attained, indicating that the antioxidant properties of each compound differ with regards to the applied method. Nevertheless, it can be argued that the number of electron-donating substituents (-OH and -OCH3) and their configuration has a significant impact on the antioxidant capacity. Undoubtedly, concerns about the reliability of these assays demand further in-depth investigations to give detailed insight into the structure and antioxidant activity relationships.