A simple experimental method for determining the steady-state shear viscosity of thermoplastic composite melts has been used. Using this method, we have determined the steady-state shear viscosity of an automotive thermoplastic composite system, namely, wollastonite-filled poly( ethylene terephthalate) (WFPET), at various temperatures and shear rates. All the viscosity properties investigated can be successfully described and predicted by a thermodynamically based rheological equation of state. The effects of composition and mixing conditions on the steady-state shear viscosity of all the WFPET investigated were also determined using this rheological equation of state.