A theoretical analysis of the solubility of interstitials in transition metal alloys is given. The change‐of‐system free energy as the interstitials are formed from the atomic state to the ionized state during the dissolving process is found to be dominated by the electronic interaction. This electronic interaction energy is characterized by the energy band structure, a rigid band model being used for the case of these bcc alloys. The minimum exhibited in the density‐of‐states curve, caused by the overlapping of subbands, gives rise to an unusual free energy change. Numerical calculations predict the interstitial solubilities in good agreement with Jones' experimental result. By comparing the solubilities of hydrogen and oxygen, oxygen should be singly ionized as an interstitial in these alloys.