Selective targeting of a redox-active ubiquinone to mitochondria within cells is a novel approach to investigate mitochondrial oxidative stress in apoptosis. The compound, named mitoQ, is a ubiquinone derivative targeted to mitochondria via covalent attachment to a lipophilic triphenylphosphonium cation. Due to the large mitochondrial membrane potential, the cation accumulates within mitochondria, where the ubiquinone moiety inserts into the lipid bilayer and is reduced by the respiratory chain. The resulting ubiquinol derivative acts as an effective antioxidant, preventing lipid peroxidation and protecting mitochondria from oxidative damage. After detoxifying reactive oxygen species, the ubiquinol moiety is regenerated by the respiratory chain, enabling its antioxidant activity to be recycled. In cell culture studies, the mitochondrially localized antioxidant protected mammalian cells from hydrogen peroxide-induced apoptosis but not from apoptosis induced by staurosporine or tumor necrosis factor-α. This suggests that mitochondrial oxidative stress may be a critical step in apoptosis induced by hydrogen peroxide but not for apoptosis induced by staurosporine or tumor necrosis factor-α. The study demonstrates that selectively manipulating mitochondrial antioxidant status with targeted and recyclable antioxidants is a feasible approach to investigate the role of mitochondrial oxidative damage in apoptotic cell death. The mitochondrial respiratory chain is a major source of superoxide, leading to oxidative damage and contributing to mitochondrial dysfunction and cell death in degenerative diseases and aging. Mitochondria are also central to activating apoptosis, and oxidative damage can lead to cell death. However, the significance of mitochondrial oxidative damage for cell death is unclear. Selectively targeting antioxidants to mitochondria allows the relative importance of mitochondrial and cytoplasmic oxidative stress for cell death to be distinguished and the contribution of mitochondrial damage to aging, diabetes, and cancer to be investigated. Derivatives of ubiquinol are promising antioxidants to target to mitochondria. In mammals, ubiquinone comprises a 2,3-dimethoxy-5-methylbenzoquinone core with a hydrophobic 45- to 50-carbon chain at the 6 position. Mitochondrial ubiquinone is a respiratory chain component buried within the lipid core of the inner membrane where it accepts two electrons from complexes I or II becoming reduced to ubiquinol, which then donates electrons to complex III. The ubiquinone pool in vivo is largely reduced, and ubiquinol is an effective antioxidant, as well as being a mobile electron carrier. Ubiquinol acts as an antioxidant by donating a hydrogen atom from one of its hydroxyl groups to a lipid peroxyl radical, thereby decreasing lipid peroxidation within the mitochondrial inner membrane. The low solubility of ubiquinone in water makes it difficult to use in vitro, and animals must be fed ubiquinone-enriched diets for several weeks to increase levels in subsequently isolated mitochondria. Therefore,Selective targeting of a redox-active ubiquinone to mitochondria within cells is a novel approach to investigate mitochondrial oxidative stress in apoptosis. The compound, named mitoQ, is a ubiquinone derivative targeted to mitochondria via covalent attachment to a lipophilic triphenylphosphonium cation. Due to the large mitochondrial membrane potential, the cation accumulates within mitochondria, where the ubiquinone moiety inserts into the lipid bilayer and is reduced by the respiratory chain. The resulting ubiquinol derivative acts as an effective antioxidant, preventing lipid peroxidation and protecting mitochondria from oxidative damage. After detoxifying reactive oxygen species, the ubiquinol moiety is regenerated by the respiratory chain, enabling its antioxidant activity to be recycled. In cell culture studies, the mitochondrially localized antioxidant protected mammalian cells from hydrogen peroxide-induced apoptosis but not from apoptosis induced by staurosporine or tumor necrosis factor-α. This suggests that mitochondrial oxidative stress may be a critical step in apoptosis induced by hydrogen peroxide but not for apoptosis induced by staurosporine or tumor necrosis factor-α. The study demonstrates that selectively manipulating mitochondrial antioxidant status with targeted and recyclable antioxidants is a feasible approach to investigate the role of mitochondrial oxidative damage in apoptotic cell death. The mitochondrial respiratory chain is a major source of superoxide, leading to oxidative damage and contributing to mitochondrial dysfunction and cell death in degenerative diseases and aging. Mitochondria are also central to activating apoptosis, and oxidative damage can lead to cell death. However, the significance of mitochondrial oxidative damage for cell death is unclear. Selectively targeting antioxidants to mitochondria allows the relative importance of mitochondrial and cytoplasmic oxidative stress for cell death to be distinguished and the contribution of mitochondrial damage to aging, diabetes, and cancer to be investigated. Derivatives of ubiquinol are promising antioxidants to target to mitochondria. In mammals, ubiquinone comprises a 2,3-dimethoxy-5-methylbenzoquinone core with a hydrophobic 45- to 50-carbon chain at the 6 position. Mitochondrial ubiquinone is a respiratory chain component buried within the lipid core of the inner membrane where it accepts two electrons from complexes I or II becoming reduced to ubiquinol, which then donates electrons to complex III. The ubiquinone pool in vivo is largely reduced, and ubiquinol is an effective antioxidant, as well as being a mobile electron carrier. Ubiquinol acts as an antioxidant by donating a hydrogen atom from one of its hydroxyl groups to a lipid peroxyl radical, thereby decreasing lipid peroxidation within the mitochondrial inner membrane. The low solubility of ubiquinone in water makes it difficult to use in vitro, and animals must be fed ubiquinone-enriched diets for several weeks to increase levels in subsequently isolated mitochondria. Therefore,