The photorefractive effect in materials such as bismuth silicon oxide (BSO) depends on photoionizing deep defect levels inadvertently present rather than controllably introduced. Using thermal stimulated conductivity measurements, a preliminary attempt has been made at associating specific levels with a particular sillenite member and impurity dopants. While many of the features prevail throughout, significant changes occur when Ge is substituted for Si to give BGO and whenp‐ (Al) andn‐type (P) impurities are added to dope BSO.