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We discuss the influence of mobile ions in liquid crystalline materials on the dc voltage-induced molecular reorientation dynamics. We investigate the conventional case of an undoped planar nematic sample aligned by thin polymeric films deposited on the electrodes. A simple model for the ion drift towards the electrodes has been developed in the limit of negligible diffusion current in order to obtain the time dependence of the internal electric field and, subsequently, the dynamics of the director distribution. This model describes the buildup of two electric charge double layers near each electrode reducing the effective voltage drop across the nematic sample. We report the measurement of the dynamics of the phase retardation in a conventional 5CB planar cell when a stepwise dc voltage above the Freedericksz threshold is applied to the electrodes. We demonstrate that our model reproduces the experimentally determined curve, thereby providing reliable values for the physical parameters used in the fittin...