Resveratrol is a polyphenolic compound frequently found in the diet, and its physiological actions have been extensively investigated. In the present study, modulation of the antioxidant and cytotoxic properties of resveratrol at different pHs by various antioxidants were investigated. To measure its antioxidant effects, resveratrol was incubated at different pHs, including 6.5, 7.4, and 8.0. Resveratrol incubated at pH 6.5 showed significantly higher DPPH radical scavenging activity, whereas resveratrol incubated at pH 8.0 did not show antioxidant effects. Resveratrol produced much higher amounts of $H_2O_2$ at pH 8.0 than 7.4. The cytotoxic effects of resveratrol on HeLa cells were significantly enhanced by several antioxidants, including superoxide dismutase, N-acetyl cysteine, glutathione, and ascorbic acid. The present results suggest that resveratrol shows anti- or pro-oxidant effects in different cellular organelles according to the pH conditions, and blocking of reactive oxygen species from resveratrol enhances its cytotoxic effects.