The microscopic structures of acetonitrile-water and DMSO-water binary mixed solvents and their influence on the solvation for solutes (some alcohols and phenol) have been studied on the basis of the cluster structures observed through a specially designed mass spectrometer. In acetonitrile-water mixtures, the water clusters were observed at water mole fractions: X-w > 0.2; on the other hand, in DMSO-water mixtures, the water clusters were observed only at much higher water mole fractions: X-w > 0.93. In the mixing processes, the water clusters were stabilized in the acetonitrile-water mixtures, whereas the DMSO clusters were stabilized in the DMSO-water mixtures. It is demonstrated that these microscopic structures directly affect the solvation for alcohols and phenol in these binary mixed solvents.