The set of acoustic mode frequencies associated with elastic constants C11, C12 and C44 have been determined from high temperature Brillouin scattering measurements on the anti-fluorite structured compound Li2S. These decrease linearly from 300 to 800 K, but above 800 K substantial reductions occur in C11 and C12. The results are interpreted in terms of normal thermal expansion and a diffuse superionic transition involving Frenkel disorder on the lithium sublattice.