An experimental investigation of the behavior of a polymer specimen for a constant stretch history has been conducted for both strip biaxial extensional flow and simple extensional flow. It had been shown previously that the stress functional of a non-Newtonian fluid under motion with constant stretch history reduces to a function of the threeRivlin-Ericksen tensors. Here it is shown that the second order approximation of a simple fluid is not capable of describing the steady-state viscosity in both types of flow under very slow motion. By considering higher orders, however, one may obtain the material constants necessary to describe steady-state non-Newtonian viscosity for both strip biaxial extensional flow and simple extension flow.