A new model of contact charging based on the change of interfacial dipole energy is proposed. The model can be used to explain the charge exchange between an insulating polymer and a metal with a thin interfacial oxide layer. The energy shift of the polymeric electronic states relative to the metal Fermi level can be associated with the change in interfacial dipole energy. The calculated change in energy for polyethylene in contact with oxidized aluminum is found to be in good agreement with data from photoemission experiments. The results are shown to be useful in improving the existing models for calculating the magnitude of the charge exchange.