In order to study the effect of the niobium instead of tantalum in barium magnesium tantalate (BMT) complex perovskite materials. The dielectric properties of (x)Ba(Ma(1/3)Ta(2/3))O-3-(1 -x)Ba(Ma(1/3)Nb(2/3))O-3 (X = 1, 0.75, 0.50, 0.25 and 0) ceramics were investigated using Fourier transform infrared (FTIR) spectroscopy from 500 to 4000 cm(-1) (15-120 THz) and pulsed terahertz (THz) time domain spectroscopy from 0.1 to 0.6 THz. The complex dielectric constant of such materials was obtained in both of the above frequency ranges. The power absorption coefficient depends is weakly on x, and increases from 0.1 to 0.9 mm(-1) and from 0.1 to 0.6 THz. The increase of loss is due to some resonance modes above I THz. In infrared reflectivity measurements, these data were analyzed via the classical dispersion theory. The spectra of Ba(Ma(1/3)Ta(2/3))O-3-Ba(Ma(1/3)Nb(2/3))O-3 were fitted by using the five resonant modes in the 15-22 THz regime. The resonance frequencies slightly shift toward higher frequencies with substitution of the Ta for Nb. The gradual increase of the Q-value in (x)Ba(Ma(1/3)Ta(2/3))O-3-(1 - x)Ba(Ma(1/3)Nb(2/3))O-3 (X = 1, 0.75, 0.50, 0.25 and 0) system in the microwave regime is can be attributed to the resonance-mode-frequency shifts. (C) 2002 Published by Elsevier Science B.V.