A study has been made of the grain‐boundary behavior in bi‐crystals of sodium chloride and magnesia held under load at elevated temperatures. Grain‐boundary sliding was observed in both materials, of a form similar to that found in metals. The sliding occurred not smoothly, but in discreet jumps followed by periods of zero movement. In sodium chloride the over‐all sliding rate was found to depend upon four variables examined: stress, temperature, atmosphere, and boundary misorientation. Two variables examined, stress and boundary misorientation, were found to influence the over‐all boundary sliding rate in magnesia. Void formation as a result of sliding was observed in a sodium chloride sample which contained a well‐defined jog in its boundary. Implications that the present results bring to bear upon general considerations of the high temperature deformation and fracture behavior of polycrystalline nonmetallic materials are discussed.