Dielectric constant (K) and loss quality (Qt) of the complex perovskite (Pb0.5Ca0.5)(Fe0.5Ta0.5)O-3 with different porosities were evaluated by the dielectric mixing rule. The observed ionic polarizabilities and loss qualities of the specimens were obtained from the dielectric constant and loss measured by the post resonant method. The theoretical ionic polarizabilities were calculated from the molecular additive rule and Clausius-Mosotti equation, and the intrinsic loss qualities were calculated from the far-infrared (Far-IR) reflectivity spectra in the range of 50-4000 cm(-1) by Kramers-Kronig analysis and classical oscillator model. For the specimens with porosity, the ionic polarizabilities modified by Maxwell's equation were more close to the theoretical values rather than those modified by Wiener's equation, and the predicted loss quality obtained from intrinsic ones and Maxwell's equation was agreed with the observed ones. (C) 2002 Elsevier Science B.V. All rights reserved.