Some water-soluble phenolic acids were investigated as antioxidants, scavengers of hydrogen peroxide (H(2)O(2)) and scavengers of 1,1-diphenyl-2-picrylhydrazyl radical (DPPH(*)). The strongest antioxidant, scavenging of H(2)O(2) and DPPH(*) radical activity was exhibited by 3,4,5-trihydroxybenzoic (gallic) acid and 1,2,3-trihydroxybenzene (pyrogallol) with three hydroxyl groups bonded to the aromatic ring in an ortho position in relation to each other. Phenolic acids with two hydroxyl groups bonded to aromatic ring in the ortho position, such as 3,4-dihydroxycinnamic (caffeic), 3,4-dihydroxybenzoic (protocatechuic) and 2,3-dihydroxybenzoic (o-pyrocatechuic) acids, showed strong antioxidant and anti-radical activity; however, it was lower than that of 3,4,5-trihydroxybenzoic acid or 1,2,3-trihydroxybenzene. 3,5-Dihydroxybenzoic (alpha-resorcylic) and 2,4-dihydroxybenzoic (beta-resorcylic) acids with two hydroxyls bonded in the meta position in relation to each other showed moderate antioxidant and low DPPH(*) and hydrogen peroxide scavenging activity. Compounds with one hydroxyl group such as 3-hydroxybenzoic, 4-hydroxyphenylacetic and 2-hydroxybenzoic (salicylic) acids, exhibited the lowest anti-radical and antioxidant activity. The results obtained show that the antioxidant and anti-radical activity of phenolic acids correlated positively with the number of hydroxyl groups bonded to the aromatic ring. The model of an ortho substitution of hydroxyl groups to the aromatic ring seems to be adequate for antioxidant and H(2)O(2) or DPPH(*) scavenging activity of phenolic acids.