The frequency-dependent impedance of magnetic tunnel junctions (MTJs) with MgO barriers was investigated. The capacitance of the MTJs switches from high to low when the relative electrode magnetizations change from parallel to antiparallel, opposite the resistance change. Additionally, for parallel magnetizations, the capacitance varies with temperature though resistance remains approximately constant. The low frequency resistance and the tunneling magnetoresistance are in agreement with dc values. The capacitance is found to be larger than the expected (geometrical) capacitance, in contrast to MTJs with Al ₂ O ₃ barriers. These results are explained by screening due to charge and spin accumulation at the interfaces.