The physical aging of three amorphous polyesters, polyethylene terephthalate, polyethylene napthalate, and polycyclohexane dimethanol terephthalate, was characterized by enthalpy relaxation via the Tool-Narayanaswamy methodology. Subtle differences in the activation energy, relaxation times, and distribution of the relaxation times are described. These data are compared to similar data for polycarbonates.