The slope of the force‐extension curve in the Hooke's law region can be measured non‐destructively, for animal fibers, at three to fifteen minute intervals. The change of this Hookean slope with time is used to follow the course of the heterogeneous reaction between wool and a reagent.Data are presented on the interaction between concentrated neutral salt solutions and wool fibers. These results indicate that wo processes occur: (a) Relatively rapid hydration or dehydration of the fiber until equilibrium with the activity of water in the salt solution is attained; (b) Relatively slow absorption of salt ions by the fiber. Very concentrated salt solutions dehydrate the fiber so completely that absorption of salt ions is not possible. Swelling experiments and experiments involving other mechanical properties confirm these hypotheses. The interaction between wool fibers and aqueous solutions of large organic molecules follows a similar pattern.