The effect on water on mechanical properties of collagen films has been studied. The S-shaped sorption isotherm is separated into an adsorption curve C₁ and a solution curve C₂. From the C₂ curve, a value of 0.8 is calculated for the Flory-Huggins inter? parameter chi₁. The dynamic shear modulus G', loss modulus G , and the loss tangent tan delta determined as functions of water content indicate two dispersions at low and at high water content. The region of water content from about 0.05 to 0.1 g/g, G' decreases suddenly, G has a peak, and tan delta increases, corresponds to the region where the C₂ component of sorption becomes detectable. Another dispersion occurs at water contents above 0.2 g/g. A composite curve can be obtained by shifting stress-relaxation curves obtained at different humidities along the log time axis. When only the C₂ component of sorbed water is taken into account, the shift factor a_c is explained by a selection of Fujita and Kishimotos' based on free-volume theory. Shift factors for the relaxation curves of wool fibers, except for an initial part at times of less than 1 sec, are described by the same equation. The parameter beta in the equation has the same value of 0.16 for both collagen and wool.