The effect of a dc electric field on the diffraction efficiency of thermally fixed holograms in Bi12SiO20 is studied. The theory predicts an increasing effect similar to that produced by the photovoltaic effect in other photorefractive materials. Experimentally, more than a 12-fold increase in the diffraction efficiency is observed for an applied field of 6300 V/cm. The decay times of the fixed holograms at different temperatures have also been studied. An activation energy of 1.4 eV is obtained for this process. The extrapolated lifetime at room temperature is longer than 104 years. Hydrogen impurities are proposed as the ions responsible for fixing.