As indicated by an Fe(II)-induced liposome peroxidation bioassay, the EtOAc extract of tart cherries (Prunus cerasus) was found to have strong antioxidant activity. Purification of this extract afforded chlorogenic acid methyl ester (1) and three novel compounds, 2-hydroxy-3-(o-hydroxyphenyl) propanoic acid (2); 1-(3', 4'-dihydroxycinnamoyl)-cyclopenta-2,5-diol (3), and 1-(3', 4'-dihydroxycinnamoyl)-cyclopenta-2,3-diol (4), as determined by their spectral data. At a 20-microM concentration, the antioxidant activities of compounds 3 and 4 were comparable to the antioxidant activities of caffeic acid, whereas compound 1 showed activity similar to chlorogenic acid. Also, these compounds showed antioxidant activities similar to the commercial antioxidants tert-butylhydroquinone and butylated hydroxytoluene. However, compound 2 was not active when tested at a 100-microM concentration.