Effects of various plant phenolics, including polyphenols, on the oxidation of oxymyoglobin were investigated. Most phenolics promoted the oxidation of oxymyoglobin at both pH 5.4 and 7.4. Potent oxidation-promoting activity was observed by several efficient antioxidant polyphenols with a catechol moiety. Therefore, effects of the catechol structure were investigated using dihydrocaffeic acid analogues. The results clarified that ortho- or para-substituted diphenol structures were important for promoting the oxidation of oxymyoglobin. Inhibition of such prooxidant activity for oxymyoglobin by dihydrocaffeic acid was also investigated. Although the required concentration was relatively higher than that of dihydrocaffeic acid, several amino acids inhibited the oxidation. Among these, cysteine was the most potent. Although cysteine alone completely inhibited oxidation at a concentration above 1 mmol/L, 0.1 mmol/L cysteine showed oxidation-promoting activity. In the presence of 0.1 mmol/L dihydrocaffeic acid, in the range of 0.01 mmol/L to 1 mmol/L cysteine, 0.1 mmol/L cysteine showed the most efficient inhibition. These results suggest the possibility of the formation of some equimolar complexes of dihydrocaffeic acid and cysteine such as 5'-cysteinyl dihydrocaffeic acid, which may be produced during the prooxidation of dihydrocaffeic acid, contributing to the inhibition of the oxidation of oxymyoglobin.