Abstract Quasielastic light scattering (QELS) due to fast ionic diffusion has been studied in β″-gallates. Intense scattering has been observed in the Tl substituted β″-gallate at room temperature. From the comparison of the scattering intensity in Tl- and Na-β″-gallates, the observed QELS is ascribed to the polarizability fluctuation of the mobile ions caused by the hopping motion of the ions. The polarization characteristics of the QELS suggest that the vacancy collisions play an important role for the light scattering mechanism in this material. Unexpectedly large intensity of the QELS observed at 155K suggests that the Tl + ions have a large hopping rate even at low temperatures.