Using a serial bilayer model we show that heterogeneous ferroelectric systems may exhibit substantial Maxwell-Wagner piezoelectric relaxation. The relaxation has its origin in the coupling of the dielectric and piezoelectric properties. The model predicts both retardation (positive phase angle and counterclockwise charge-pressure hysteresis) and relaxation (negative phase angle and clockwise charge-pressure hysteresis) of the longitudinal direct piezoelectric coefficient. The theoretical results are confirmed experimentally in ferroelectric ceramic-ceramic bilayers and in a single-phase ferroelectric Aurivillius compound ceramic in which neighboring grains with strongly anisotropic properties may behave as basic Maxwell-Wagner units. (C) 2001 American Institute of Physics.