A new theoretical approach to the problem of describing the nonlinear behavior of non‐Newtonian materials is presented. The theory is based upon the assumption that the nonlinear characteristics can be associated with some structural change of the material, and that reaction kinetics can be applied to this breakdown. The theory allows treatment of time‐dependent (thixotropic) materials, and in a limiting form (equilibrium) can be applied to time‐independent materials. The theory has been applied to a variety of materials, including polymer latex solutions, polyethylene polymer melts, and a heavy thixotropic mineral oil. The results are gratifying.