The solubility of ß-cyclodextrin (ß-CD) in water has been measured by a refractive-index method at 15–30°. Evidence was obtained that the same, solid ß-CD hydrate phase is present in this temperature range. The formula of the hydrate was shown to be C42H70O35(12.0 ± 0.5)H20. A method for preparation of this ß-CD hydrate is described. The relations between refractive index and concentration of ß-CD are given. The calculated enthalpy of solution ¿H50l = 7.094 ±0.021 kcal. mole-1. This value will only slightly be influenced by the number of moles of water per mole of ß-CD in the hydrated solid. The solubility of ß-CD as a function of temperature can be described by 2.303 log10 m = (3.88±0.02) - (3583±18)/T where m = mole fraction of anhydrous ß-CD at saturation, and T = absolute temperature.