The protection of substrate by an antioxidant is modeled as the redox reaction of two reduced and one oxidized components. The equilibrium concentration of the reduced form of substrate is calculated as a function of formal potentials and total concentrations of oxidant and antioxidant. The conditions are determined under which the oxidative damage is limited to 10% of the total concentration of substrate. (C) 2015 Elsevier B.V. All rights reserved.