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The development of physical aging in four amorphous poly(aryl ether ketone)s at temperatures below the glass transition temperature, Tg, was studied using differential scanning calorimetry (DSC). The change in enthalpy lost on aging, determined by scanning through Tg, was used to determine the kinetics of the aging process through fitting to the Cowie–Ferguson model. The kinetics of aging were found to depend on the ketone–ether ratio as well as the substitution pattern of the polymers. The maximum enthalpy lost at equilibrium, Δ H∞, was found to be a linear function of the change in heat capacity at Tg, Δ Cp(Tg), and the degree of supercooling, Δ T. A composite curve incorporating Tg values from physical aging and dynamic mechanical studies, covering 15 decades, could be fitted to the Volger–Fulcher equation and demonstrated that physical aging is an extension of the glass forming process.