An investigation was made of the defect characteristics of single‐crystal Bi12SiO20 (BSO) grown by both Czochralski (Cz) and hydrothermal transport methods. Only Cz‐grown BSO is photorefracting using a coherent pumping source centered around 500 nm, as undoped hydrothermal BSO is transparent throughout the visible spectrum. Thermally stimulated current (TSC) studies in conjunction with temperature‐dependent optical‐absorption measurements and room‐temperature photoconductivity data all indicate that the hydrothermal material is near intrinsic in terms of its low defect content. TSC measurements made below room temperature indicate that concentrations of traps of activation energy ≪0.7 eV are a factor of 103 smaller in hydrothermal than in Cz BSO. At least six different defects were identified in the TSC measurements. Temperature‐dependent optical‐absorption measurements indicate two Urbach band tails for Cz‐grown materials that are not observed in the hydrothermal materials. Cz material of lower purity also possesses an impurity band tail which can be observed through temperature dependent optical absorption measurements. Comparison of Cz material with hydrothermal BSO of similar impurity content suggests that the BiSi defect responsible for the 500 nm absorption may be complexed with an impurity such as Fe or V. In the absence of this defect, however, these impurities have no effect on the absorption. An additional defect in the TSC data is also related to a transition metal impurity. The results indicate that the photoconductivity associated with the photorefractive effect in Cz material must proceed via a trap‐hopping conduction mechanism that is missing in the intrinsic material.