The concentration and the temperature dependencies of H-1 and C-13 chemical shifts in NMR of aqueous acetone mixtures were studied, together with the concentration dependence of the frequency of the C-H stretching vibration of acetone in IR spectra. H-1 and C-13 chemical shifts were measured at 1 degrees C, 23 degrees C, and 48 degrees C by the external double reference method using a capillary with a blown-out sphere at the bottom for tetramethylsilane as the external reference substance. By this method, it is possible to determine the volume magnetic susceptibility of a sample solution at each temperature, for which the observed chemical shifts may be corrected exactly. Thus, we revealed the detailed electronic polarization in acetone as well as water as functions of concentration and temperature. On diluting acetone with water, the chemical shift of water protons, (H2O)-H-delta, is 3 ppm at the mole fraction of water X-w = 0,05 and increases to the value for pure water, ca. 5 ppm, at X-w =0.96, with increasing X-w. In the region of X-w>0.96, (H2O)-H-delta is Slightly larger than the value, indicating the presence of more polarized water species than pure water. The chemical shifts of C-H proton, (CH3)-C-delta, and C-H carbon, (CH3)-C-delta, also increase slightly with increasing X-w up to X-w = 0.96. The frequency for the C-H vibration of acetone, v(C-H), increases from the value for pure acetone, 3005 cm(-1), to 3013 cm(-1) at X-w = 0.96, while it decreases sharply with further increase in X-w. These results of IR and NMR measurements show that the hydration of acetone accompanies electronic redistribution in the C-H bonds in cooperated with the change in the polarization of the surrounding water molecules, and that two different types of hydration of acetone are predominant in different concentration regions, X-w<0.96 and X-w>0.96. In the region of X-w<0.96, the results can be explained satisfactorily if we consider that a part of the electron about the C-H proton is pushed out into the C-H bond due to a repulsive interaction between the C-K hydrogen and water oxygen. In the region of X-w>0.96, we can interpret the results well by considering that the pushing by the water oxygen becomes strong enough to induce the polarization of the C-H bonds compared to the pushing at X(w)less than or equal to 0.96. Since the polarization of the C-H bond was found to increase with decreasing temperature, the repulsive interaction seems to have the property of hydrogen bonding and to be denoted as C-H ... OH2(... OH2)(n), where OH2(... OH2)(n) expresses water molecules hydrogen-bonded cooperatively and responsible for the more polarized water than pure water. The ratio of water to acetone seems to be a predominant factor to cause the transition of the hydration state from the repulsive interaction to hydrophobic hydration of acetone.