In this study the effect of additional hydroxyl or metoxyl substituent, at ortho position to OH group, on the radical scavenging antioxidant activity and on pKa as well as on electron and hydrogen donating abilities of structurally related 4-hydroxybenzoic acid, 4-hydroxycynnamic acid, 4’,3,5,7-tetrahydroxyflavone, and 4’,3,5,7-tetrahydroxyanthocyanidins was investigated experimentally and also based on computer calculations. Antioxidant activity of the polyphenolic compounds over a wide pH range (4.5-9.5) was quantified using the modified TEAC assay. The results show significant pH-dependence on the radical scavenging antioxidant activity of polyphenolic compounds studied and reveal that important factors influencing TEAC antioxidant activity of polyphenolic antioxidants are not only the number and position of hydroxyl moieties in the molecule, but also their protonation state, influenced by the pKa of hydroxyl moieties, the pH of the surrounding medium, the effect of intramolecular hydrogen bridges, and O-methylation of hydroxyl groups. The results obtained demonstrate the applicability of the modified TEAC assay for studying the pH-dependent effects on the radical scavenging antioxidant activity of (poly)phenolic antioxidants. Moreover, this assay allows the determination of the TEAC value for the neutral form as well as for deprotonated form of (poly)phenolic compound, which is a proper basis for studying the mechanism of antioxidant action by comparison to calculated electronic parameters.