A theoretical model of the grain boundary potential is developed by considering the presence of deep level impurities and a generalized interface state model. It is seen that the grain boundary potential is maximum at a certain value of the deep level density. This happens when the type of deep level impurities is opposite to that of shallow impurities. For similar types of deep and shallow impurities, the potential barrier remains constant over a wide range of impurity concentration for lower values of activation energy, but decreases considerably when the activation energy is increased. Copyright © 1994 WILEY‐VCH Verlag GmbH & Co. KGaA