The Nrf2 transcription factor and its cognate antioxidant response element (ARE) regulate expression of antioxidant proteins and other cytoprotective proteins. Therapeutic activation of this system is under investigation in numerous pre-clinical and clinical trials for a range of common chronic diseases and their prevention. This system responds to oxidative stress, such as that caused by H2O2, as well as to many electrophilic small molecules, including sulforaphane from cruciferous vegetables. Various Nrf2 activators can both generate oxidative stress and behave as electrophiles, for example, oxidation of phenolic compounds to the quinone form. Our objective is to understand the interplay of reactive oxygen species (ROS) and electrophiles in the activation of the Nrf2/ARE system. Our model system utilizes sulforaphane as the electrophile and the ROS-generating oxidizable diphenol, 2,5-di-tert-butylhydroquinone (dtBHQ), whose oxidized form is sterically blocked from reacting with nucleophiles. Our data indicate that H2O2 generated from dtBHQ oxidation synergistically enhances sulforaphane-induced ARE-regulated gene expression. Paradoxically, co-treatment with H2O2-scavenging catalase lowers gene expression levels to below that of sulforaphane alone, implying ROS other than H2O2 inhibit gene expression in some manner. Correspondingly, we find sulforaphane-induced Nrf2 protein levels not only fail to increase in the presence of dtBHQ, but rather they are significantly reduced. To confirm that other forms of ROS are responsible for this inhibition, we tested the effect of the catalyst MnTMPyP, which primarily degrades peroxynitrate and superoxide, and subsequently also inhibits hydroxyl radical formation. Cells were treated with sulforaphane, dtBHQ and either catalase or MnTMPyP. We find MnTMPyP but not catalase rescues Nrf2 protein levels, and that it increases the response of the ARE reporter. Therefore, there is a Dr. Jekyll and Mr. Hyde effect of ROS on electrophile-mediated ARE activation, with H2O2 playing a beneficial role, and other species, possibly the highly oxidizing species peroxynitrate or hydroxyl radical, causing inhibition.