Material functions of two engineering plastics [a poly(phenylene ether) and a poly(ether imide)] were characterized, including the shear viscosity, first normal stress coefficient, storage and loss moduli, growth and relaxation of shear stress, and first normal stress coefficient and relaxation moduli. The oscillatory shear and relaxation moduli data were employed to determine the temperature‐dependent parameters of Wagner model. Various material functions, which were determined on the basis of this model in conjunction with the fitted parameters agreed reasonably well with the experimental results. The reported data and parameters should facilitate a better understanding of the processability characteristics of these two engineering plastics.