Surface potential decay after charge deposition by corona or contact has been studied for epoxy samples and compared with polarization measurements. We show that the classical interpretation of the potential decay in terms of the injection of the deposited charge into the volume of the insulator does not account for our results, which can be explained by the effect of slow volume polarization alone. The return potential phenomenon also finds a satisfactory interpretation using this model. The polarization processes involved are the same as for absorption currents. Hence the model may apply to a large variety of other insulating materials.