Members of Polygalaceae are known to contain a variety of different polyphenolic compounds such as xanthones, flavonoids, and biphenyl derivatives. Here, we report the isolation and structural characterization of two new phenol derivatives, named alpestrin (= 3,3',5'-trimethoxy[1,1'-biphenyl]-4-ol; 10) and alpestriose A (= 6-O-benzoyl-1-O-{6-O-acetyl-3-O-[(4-hydroxy-3,5-dimethoxyphenyl)prop-2-enoyl]-beta-D-fructofuranosyl}-alpha-D-glucopyranoside; 11), and of four known compounds (12-15) from the MeOH extract of Polygala alpestris. The relative in vitro antioxidant activities of these compounds, in comparison with other phenolic substances from Polygala vulgaris, were evaluated by means of the Briggs-Rauscher (BR) oscillating reaction, a method based on the inhibitory effects of antioxidant free-radical scavengers. The experimental antioxidant-activity values (relative to resorcinol as a standard) were compared with those calculated on the basis of the bond-dissociation enthalpies. The structure/activity relationships for the compounds examined are also discussed.