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 center dot and HOO center dot radicals in water and, for a lipid-mimetic environment, pentyl ethanoate solvent, OH using kinetic calculations. It was found that the overall reaction rate constant of the HO center dot radical scavenging is more than 7 times higher in aqueous (8.98 x 10(9) M-1 s(-1)) than in apolar (1.22 x 10(9) M-1 s(-1)) media. However, HOO center dot scavenging was 35 times faster in apolar media (1.00 x 10(5) M-1 s(-1) vs 2.80 x 10(3) 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 center dot scavenging, while HAT described the HOO center dot scavenging. IH was shown to be one of the most active antioxidants in lipid environment, an essential characteristic for the protection of biological systems.