The salt-induced phase separation was, for the first time, directly observed at a molecular level by small-angle neutron scattering (SANS) experiments performed at room temperature on acetonitrile-D2O and acetonitrile-D2O-NaCl mixtures as a function of acetonitrile mole fraction, X-AN, and NaCl concentration, respectively. Aggregation or microheterogeneity in the acetonitrile-D2O mixtures was most enhanced at X-AN = 0.3-0.4 with Debye correlation lengths L-D similar to 19 Angstrom. In the acetonitrile-D2O-NaCl mixtures at X-AN = 0.2 the aggregation increased with NaCl content and reached a plateau of L-D similar to 20 Angstrom.