Solubility of substitutional impurities in decomposing interstitial solid solutions with a cubic structure has been calculated depending on the temperature and decomposition parameter. It is shown that near the dome of the decomposition curve the passage of the system into a two-phase state leads to an increase in the solubility. At large values of the decomposition parameter the change in the solubility upon the passage into the two-phase state occurs in a a more complex manner; depending on the values of the system parameters, both an increase and a decrease in the solubility can be observed upon the decomposition.