Differential scanning calorimetry (DSC) was used to measure the relaxation enthalpy of glassy amorphous poly(ether imide) (PEI) at three high isothermal aging temperatures between 190 and 200 degrees C. A three-parameter model based on the Williams-Watts relaxation function was utilized to describe the aging process of PEI, and nonlinear regression approach was used to estimate the model parameters. The prediction according to this model was found to be more accurate in comparison with the two-parameter model based on the same Williams-Watts relaxation function. Furthermore, the peculiar aging behavior at the highest aging temperature of 200 degrees C was explained from the molecular view point of transition from sub-T-g beta-relaxation to alpha-relaxation near T-g.