Ba2LaTi2Nb3O15 exhibits a smeared maximum of permittivity, characteristic of classic relaxor behaviour, with a peak shift from 185 K at 100 Hz to 300 K at 1 GHz. Ba2NdTi2Nb3O15 undergoes a first order ferroelectric phase transition at 389 K and Ba2La0.5Nd0.5Ti2Nb3O15 exhibits both a ferroelectric phase transition at 274 K and relaxor behaviour at higher temperatures. All three ceramic systems were investigated across a broad frequency (100 Hz–1 THz) and temperature (10–500 K) range. Slowing down of the dielectric relaxation across the whole investigated temperature range from the THz and microwave regions to below 100 Hz was observed on cooling in the relaxor ferroelectric Ba2LaTi2Nb3O15. The mean relaxation frequency obeys the Vogel–Fulcher law with freezing temperature of 40 K. In Ba2NdTi2Nb3O15 and Ba2La0.5Nd0.5Ti2Nb3O15 the relaxations soften only in the paraelectric phase down to the 1 GHz and 100 MHz range, respectively. Below Tc the relaxations vanish. The origin of the dielectric relaxations is discussed on the basis of structural data.