The luminescence spectra and decay curves of Sb3+-doped Cs2NaMCl6 (M=Sc,Y,La) are presented. In these compounds the 5s2 ion is octahedrally surrounded by six chloride ions like the s2 ions in alkali halides. Therefore, they can be considered as an ideal model system. It appears that the Sb3+ luminescence in Cs2NaMCl6 is comparable with the luminescence of the isoelectronic s2 ion In+ in potassium chloride. The emission consists of one band moving to lower energies from scandium to lanthanum, suggesting that the Stokes shift increases when the space available for the Sb3+ ion increases. The excitation spectra can be explained by assuming a dynamical Jahn-Teller effect to be operative in the excited state of the Sb3+ ion. The consequences of this interaction are discussed.