Since the use of radioiodine for thyroid therapy in the form of an oral solution has risks associated with contamination, the need to develop suitable Na131I capsules has attracted significant attention. A series of different mass combinations of excipients was prepared using ascorbic acid, sodium thiosulphate, mannitol, maltose, magnesium sulphate, lactose, dextrose, sodium phosphate, and sucrose. These compounds were tested for their volatility and oxidation profile. Two of the excipient mixtures were selected to prepare capsules due to their high stability and low volatility. The experiments performed to evaluate a new excipients mixture are described in this manuscript.