The reactivity of a series of indole derivatives was assessed in the following systems: (i) oxidation of the indole derivatives induced by the thermolysis of 2,2'-azobis-(2-amidinopropane) (ABAP); (ii) oxidation of cumene induced by the thermolysis of 2,2'-azobis-(2-methyl propionitrile) (AIBN); (iii) lysozyme inactivation induced by the thermolysis of ABAP and (iv) brain homogenate autoxidation. In systems (ii) to (iv), addition of the indole derivatives decreases the rate of the process. The data obtained indicate that common factors (i.e., oxidation potential and presence of N-H bonds) control the reactivity of the indole derivatives in the four systems considered. However, in the brain homogenate autoxidation, hydrophobicity is an additional factor that affects the efficiency of antioxidants, as illustrated by Q1/2 values (the concentration of additive required to decrease the autoxidation rate to one half that observed in the absence of additive) of 0.1 mM and much greater than 8 mM for 3-methylindole and tryptophan, respectively.