We have observed modulation of the dark current in Bi12SiO20(BSO) as a function of prior irradiation, and a time‐varying absorption of radiation that is dependent upon prior irradiation. As a result of these observations and the previously reported observations of photoconductivity, photorefractivity, photosensitivity, and mobility modulation, a model theory of transport has been developed that requires multiple sources of defect/impurity energy levels. A hopping band exists in the forbidden energy gap where these levels coincide. Transport via exchange of charge is possible here, analogous to conduction via hopping in the presence of impurity bands or dense concentrations of defects. Traps capable of containing immobile charge carriers occur where these various levels do not coincide. Large mobility variations can occur as a result of photon‐induced modulation of the Fermi level. Much of the phenomena observed in BSO can be explained through interaction between the multiple defect/impurity energy levels.