Indolinonic hydroxylamine (IH) is a new-generation artificial antioxidant that, due to its ability to fractionate into apolar environments, is considered for prevention against lipid peroxidation. For this reason, it is important to understand, and compare, its activity in polar and nonpolar environments. In this study, the antioxidant activity of IH has been evaluated against HO• and HOO• radicals in water and, for a lipid-mimetic environment, pentyl ethanoate solvent, using kinetic calculations. It was found that the overall reaction rate constant of the HO• radical scavenging is more than 7 times higher in aqueous (8.98 × 109 M-1 s-1) than in apolar (1.22 × 109 M-1 s-1) media. However, HOO• scavenging was 35 times faster in apolar media (1.00 × 105 M-1 s-1 vs 2.80 × 103 M-1 s-1). In a lipid environment, the HAT mechanism was favored for the antioxidant activity for both radical species, whereas in aqueous solution the SET mechanism defined the HO• scavenging, while HAT described the HOO• scavenging. IH was shown to be one of the most active antioxidants in lipid environment, an essential characteristic for the protection of biological systems.