We have measured ac complex impedance and thermal depolarization current for aged(K0.50Na0.50)2(Sr0.75Ba0.25)4Nb10O30ceramic samples to investigate the influence of mobile space charges on the ferroelectric properties. The impedance behavior was analyzed using an equivalent circuit composed of a capacitor connected in series with a parallel resistor-capacitor circuit. The dielectric relaxation is found to involve two mechanisms; one is attributed to the hopping process of the space charges bound inside grains and the other is associated both with the intrinsic polarization of the ferroelectric material and with the space charges at the grain boundary layer. The experimental results are also discussed in terms of the aging mechanism. It is suggested that the space charges bound inside grains play an important role in the aging process of the ferroelectric ceramics. ©1997 American Institute of Physics.