An empirical equation is introduced to calculate the absorption/excitation energy of Bi3+ pairs in oxidic compounds from the knowledge of their crystal structure. The method is based on recent reports that describe the optical processes within the pair as resulting from an intervalence charge transfer of the type Bi3+, Bi3+ → Bi2+, Bi4+. The data are used to locate the vacuum referred binding energy of the electron in the 2P1/2 ground state of Bi2+ ions in few oxidic compounds.