Thechain-breaking antioxidant activities of reduced form of novel type ofgeroprotectors, mitochondria-targeted quinones (QH2) havequantitatively been measured for the first time. To this end, the chainperoxidation of methyl linoleate (ML) in Triton micelles was used as akinetic testing model. The studied QH2 were lipophilictriphenylphosphonium cations conjugated by an aliphatic linker to anantioxidant, i.e. a ubiquinol moiety (MitoQH2) or plastoquinolmoiety (SkQH2). The antioxidant activity was characterized bythe rate constant k1 for the reaction between QH2 andthe lipid peroxyl radical (LO2·) originatedfrom ML: QH2 + LO2· → HQ· + LOOH. Allthe tested QH2 displayed a pronounced antioxidant activity. Theoxidized forms of the same compounds did not inhibit ML peroxidation. Thevalue of k1 for SkQH2 far exceeded k1 forMitoQH2. For the biologically active geroprotectors SkQ1H2,the k1 value found to be as high as 2.2 × 105 M-1s-1,whereas for MitoQH2, it was 0.58 × 105 M-1s-1. The kineticbehavior of QH2 suggested that SkQ1H2 can rathereasily diffuse through lipid-water microheterogeneous systems.