The effect of extrinsic traps on the charge transport in organic semiconductors has been investigated. An analytical model describing hopping transport with traps is formulated on the basis of percolation theory. The results show that the presence of a trap distribution with energy offset and width different from that of the intrinsic density of states does not change the basic phenomenology of hopping transport, as revealed by the temperature dependence of the conductivity at high temperature. However the traps may significantly affect the transport at low temperature. The relation between trap concentration and conductivity is discussed. The model predictions are in good agreement with experimental results. (C) 2007 Elsevier Ltd. All rights reserved.