[en] The concentration fluctuation of acetonitrile-water and acetonitrile-water-MCl (M=Li⁺, Na⁺, and K⁺) mixtures has been investigated by using small-angle neutron scattering (SANS) technique. At 298 K, the concentration fluctuation of acetonitrile-water mixtures is most enhanced at acetonitrile mole fraction of x_AN=0.3-0.4. When the temperature decreases to 279 K, the concentration fluctuation of the mixtures at x_AN=0.3-0.4 drastically increases. For acetonitrile-water-MC1 mixtures, the concentration fluctuation more easily increases in the order of NaCl>KCl>LiCl. This order agrees with that of salt-induced phase separation of acetonitrile-water mixtures. (author)