The local structures of acetic acid in protic and aprotic polar solvents have been studied by Raman spectroscopy and ab initio calculations with the self-consistent reaction field (SCRF) method. As acetic acid is diluted in water, the C=O stretching Raman band of acetic acid becomes broader and shows a higher wavenumber shift from 1666 to 1710 cm-1, which arises from the generation of acetic acid microphases. In the region of 0.001 ≤ χA (acetic acid mole fraction) ≤ 0.2, both the peak position and the bandwidth of the C=O band are hardly changed, indicating that the acetic acid microphases exist even in the diluted solution at χA = 0.001. In alcohols (methanol, 1-butanol, and 1-hexanol), the spectral changes in the C=O band with the dilution are almost the same as those observed in water, suggesting that the same acetic acid microphases are formed in the alcohol solutions at χA ≥ 0.001. In acetonitrile, however, the spectral changes are apparently different from those in the protic solvents: two higher wavenumber C=O bands at 1725 and 1754 cm-1 appear in the region of 0.001 ≤ χA ≤ 0.3. From the ab initio SCRF calculations, we assign the 1725 and 1754 cm-1 bands to the cyclic dimer consisting of acetic acid and acetonitrile monomers and to the noncomplexed acetic acid monomer, respectively. Such two bands are also observed in other nitriles and ethers, suggesting that the monomeric molecules are preferentially formed in aprotic polar solvents. From these results, it is concluded that binary solutions of acetic acid and the protic solvents do not get homogeneously mixed even in the low acid concentration region of χA ≥ 0.001, while homogeneously mixed states at molecular levels occur in binary solutions of acetic acid and the aprotic polar solvents when the acetic acid mole fraction is small. We discuss the empirical rules about the mixture states at molecular levels on the basis of the results obtained.