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In acetonitrilewater mixtures, dynamical processes of hydrophobic substrates, such as deprotonation of excited-state 2-naphthol and hydrolysis of tert-butyl chloride, hardly occur at water mole fractions (x,) less than 0.8. In contrast, the reaction rates increased markedly with increasing x, for x, > 0.8. These results were correlated with properties which reflect the solvent clustering conditions, s u c h as partial vapour pressure and IR absorption of acetonitrile-water mixtures. IR absorption of water molecules due to the antisymmetric 0-H stretching vibration showed a step-wise change of t h e absorption peak position suggesting the presence of four different solvent structures in the regions 0 < x, < 0.2, 0.2 < x, < 0.5, 0.5 < x, < 0.8 and 0.8 < x, < 1.0. Mass spectrometric analyses of liquid fragments of acetonitrile-water mixtures and the solutions with phenol, benzyl alcohol and 2-n-butoxyethanot showed clustering conditions characteristic of the water mole fractions in the mixtures. In 0.8 < x, < 1.0 the hydrate clusters of phenol and benzyl alcohol increased very quickly with increasing x,. In x, < 0.8, the signals of the hydrate clusters of the solutes were very weak, while the acetonitrile hydrates increased with decreasing x,. The observed mass spectral change with varying x, is discussed in relation to the structure of acetonitrile-water solutions and its effect on t h e dynamical processes of hydrophobic substrates.