The exposure of blood serum to reactive oxygen species creates free radicals and damages the structure of biomolecules. It causes proton NMR relaxation times to change over time. In aqueous protein solutions, T1 and T2 decay curves were fitted by single exponential components. Following addition of hydrogen peroxide to blood serum which contained endogenous antioxidants, relaxation times initially decreased then increased towards initial values. The character of these curves and their fitting parameters depend on kinetics of oxidation processes. We hope that MR imaging may help to investigate these important processes also in vivo.