Sulfur dioxide is a controversial preservative used in winemaking. Given its potential toxicity, it is of great interest to find alternatives aimed at replacing or reducing sulfur dioxide. The objective was to assess the antioxidant effects of a grapevine-shoot extract with 99% of stilbenes (ST-99), its major compounds (trans-resveratrol and trans-ε-viniferin) and their mixture (1:3.9) by measuring reactive oxygen species (ROS) and glutathione (GSH) levels in vitro. Their ability to protect against or reverse the effects of H2O2 on cells were also studied. The results showed that ST-99, followed by trans-ε-viniferin and the mixture, were able to reduce ROS levels, increased GSH content and exhibited antioxidant ability against an induced oxidative stress. Trans-resveratrol significantly reduced ROS content only at the highest concentrations. ST-99 at non-cytotoxic concentrations is more effective than the other compounds, which might be attributed to increased levels of GSH. The results suggest a promising use of ST-99.