Abstract Creep studies and differential scanning calorimetry (DSC) indicate that physical (thermal) aging affects viscoelastic properties of polyether ether ketone (PEEK) at all achievable levels of crystallinity. The phenomenon has been observed over a range of crystallinity (2%–33% by volume) at several test temperatures. When the glass transition temperatures (T g ) are different, even for the same undercooling (T g − T) , the parameters are significantly different. Dependence of enthalpy relaxation on aging time was obtained from DSC thermograms for the amorphous polymer. The reported results provided aging parameters which are required for making long term predictions of creep behavior.