A well-known problem in packaging of micromechanical devices by anodic bonding is the resulting bow of the devices. The present paper deals with a method to change this bow after bonding by annealing the wafers below the glass transition region. By means of laser optical curvature measurement the change of the bow during several annealing procedures has been observed in situ. It is shown that the bow of bonded wafer pairs can be reduced, removed, and even induced in the opposite direction. The observed bowing behavior is explained by the structural relaxation of borosilicate glass.