Abstract The room-temperature elastic constants of a number of hexahalometallate A 2 MX 6 single crystals [K 2SnCl 6, K 2ReCl 6, (NH 4) 2SnCl 6, (NH 4) 2SnBr 6, (NH 4) 2SiF 6, Rb 2SnBr 6, K 2SeBr 6, (NH 4) 2TeBr 6, K 2PtBr 6 and (NH 4)2PtBr 6] have been measured either by Brillouin scattering or by the ultrasonic pulse echo overlap technique. Refractive indices have also been determined. These antifluorite structure compounds contain large MX 2− 6 ions and the interionic spacings are much greater than those of the alkaline-earth fluorite structure halides: their elastic stiffnesses are correspondingly smaller. Hydrostatic pressure derivatives of the elastic stiffness constants have been measured for K 2SnCl 6, (NH 4) 2SnBr 6 and (NH 4) 2SnCl 6 and are found to be positive; there is no marked softening of the long-wavelength acoustic-phonon modes at room temperature. The vibrational anharmonicities of these long-wavelength modes are discussed in terms of the acousticmode Grüneisen parameters, which are compared with the thermal Grüneisen parameters. For K 2SnCl 6 a mean of optic- and acoustic-mode Grüneisen parameters is shown to correlate well with the thermal Grüneisen parameter.