Acoustic and dielectric properties of Pb(Mg1/3Ta2/3)O3 (PMT) and Ba(Mg1/3Ta2/3)O3 (BMT) single crystals with B-site disorder were obtained in a wide temperature range by micro-Brillouin scattering and dielectric spectroscopy. The temperature dependences of both the dielectric constant and the Brillouin shift of the longitudinal acoustic (LA) mode of BMT showed that there is no structural phase transition below 860 K, consistent with thermal expansion measurements. A low dielectric constant Є∼20 of BMT reflected the low-level flux contamination. PMT crystals showed frequency-dependent dielectric maximum, significant softening of the Brillouin shift of LA mode deviating from the high-temperature linear behavior. The temperature dependence of the maximum relaxation time could be obtained by combining dielectric and acoustic data, which followed the Vogel–Fulcher law with a freezing temperature of 119±6 K. This temperature coincides with that estimated from the frequency dependence of the temperature of the dielectric maximum, which is the evidence of the real freezing of the relaxation-time spectrum of PMT at a finite temperature. © 2003 American Institute of Physics.