Radiolytic stability is one of the main requirements of the substances that are used in the chemistry of nuclear cycle or in the radiopharmaceutical chemistry. Herein, we proposed an approach for the prediction of radiolytic stability by the estimation of the molecular reactivity. The DFT calculations of the atom-wise reactivity descriptor were made for a number of organic molecules. The theoretical simulations were validated by the experimental data. We irradiated the molecules by gamma-radiation and studied the products of radiolysis and changes in the molecular concentration by HPLC-MS analysis. The importance of the inclusion of the conformational influence and the steric accessibility in the calculation is shown in this study. We presente a new chemical reactivity descriptor (CRD) and recommend using CRD as the new quantitative estimation of the reactivity. A good correlation between the CRD and the constants of the radiolysis was obtained.