Autocomelation spectra for ion-density fluctuations in molten RbBr are evaluated over a wide range of wave number (0.3 Å−1≤q≤ 7 Å−1in the microscopic region of wave number and frequency. using a memory-function theory which neglects thermal fluctuation effects. The results are critically compared with data obtained by the Argonne group through neutron inelastic scattering experiments and computer simulation. Agreement is good in an intermediate range of wave number (1 Å−1≤q≤ 4 Å−1) but progressively worsens outside this range. In particular, the need for a more complete generalized-hydrodynamic theory at long wave-lengths, inclusive of couplings to entropy fluctuations, is underlined.