Switching behavior of 90° domains in epitaxial Pb(Zr0.32Ti0.68)O3 thin films under applied bias voltage was investigated in situ using synchrotron x-ray diffraction, and contribution of the switching to ferroelectric P–E hysteresis curve could be estimated. The electric field in the region illuminated by x ray was made confined exactly normal to the film/substrate interface by patterning an isolated capacitor (1×1 mm2) and etching off the remainder in order to eliminate mechanical constraint from nonswitching region. The portion of polarization taken up by 90° domain reversal was separated from 180° domain switching after measuring the changes in relative intensity ratio of PZT (001) and (100) reflections, which exhibited hysteresis behavior depending on applied voltage. Within the experimental region of electric field up to 24 kV/mm, maximum 27.8% of 90° domains were reoriented, which corresponds to ∼2% contribution to total polarization. © 2001 American Institute of Physics.