The hydration numbers: of cellulose ethers dissolved into aqueous solution were determined using extremely high-frequency dielectric spectroscopic methods, up to 50 GHz, in which the frequency range of the major dielectric dispersion of free water molecules is clearly observable. Methyl cellulose, hydroxypropylmethyl cellulose, and hydroxyethylmethyl cellulose samples at relatively high degrees of substitution by methyl groups ranged from 1.4 to 1.9, which are an essential reason for water solubility, and additional low molar substitution by hydroxypropyl or hydroxyethyl groups from 0 to 0.25 bearing molar masses (M-w) ranging from 20 X 10(3) to 300 X 10(3) was examined in this study. All the samples dissolve well into cold water below room temperature, ca. 25 degrees C; however, their aqueous solutions show distinct cloud points at high temperatures, such as 38-53 degrees C, depending not so much on the M-w values but on the substitution condition substantially. The determined hydration numbers per glucopyranose unit (n(H)) for the cellulose ethers in aqueous solution were more than 12 at 10 degrees C and were slightly influenced by the substitution condition. The cellulose ethers showed remarkable dehydration behavior with increasing temperature; then, the n(H) values significantly decreased to the value of ca. 5 almost irrespective of the substitution condition and M-w in the vicinities of then lower critical solution temperatures (LCSTs) controlled by the substitution condition. These observations strongly suggest that the value of n(H) = 5 near each LCST is the minimum critical quantity for the cellulose ethers to be dissolved into water, and when the value of n(H) is less than the value at higher temperatures than cloud points, aqueous solutions of cellulose ethers become opaque.