The description of the flow mechanism of nonlinear viscoelasticity is based on what is known and on what can be measured by experiment. Such materials harden or soften under stress. These processes cannot go on indefinitely, but must result finally in failure. Their deformations are both reversible and irreversible, hence stresses and entropy changes at constant temperature consist also of reversible and irreversible parts. Experiments are generally carried out by applying an external force and measuring the resulting deformation. For the nonlinear case, these quantities are not knowna prioriin terms of stress and strain components, but the problem can be simplified by treating nonlinear behavior as a special case of linear behavior. It is shown that, where the normal stress‐strain or strain rate relationships are constant for the linear case, they are also constant for the nonlinear case if the strain or stress rate consists of the sum of reversible and irreversible parts. The upper limit of this constant relationship coincides with the stress at which a nonlinear material has its ultimate strength. At greater stresses, the material is in a region of instability. The validity of this approach is demonstrated by data on a bituminous paving mixture (typical of hardening) and on several molten polyethylenes.