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.