The dielectric constant ϵ' and dielectric loss ϵ" of human cortical bone have been measured for frequencies of 1, 10, 50 and 100 kc and for hydrations (h) from 0 to 65 mg-H2O/g-bone. The absorption isotherm has also been determined. Curves of ϵ' and ϵ"vs h are found to alter sharply at a particular value of the hydration, h_c, defining the `critical hydration' of bone. The quantity h_c, ranges from 37 to 48 mg-H₂O/g-bone, depending on the density of the sample. It is interpreted as the amount of water necessary to occupy the primary absorption sites in bone, i.e., as the bound water of the system. The relationship of h_c with previously reported values of the bound water in hide collagen and synthetic hydroxyapatite is discussed.