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The double-melting behavior of poly(aiyl ether ketones) (PAEKs) was detected by differential scanning calorimetry (DSC) experiments. The samples tested were poly (ether ether ketone) (PEEK) and poly(ether ketone (PEK), which have been crystallized from the glassy state at different crystallization temperatures Tc. The size and location of the lower peaks on DSC traces were affected by Tc, provided that Tc > Tg. An influence of Tc on the mechanical behavior of stretched PEEK and PEK (uniaxial deformation mode at 200 °C) was also observed. The perfection of crystalline structure was thought to be a reason for this influence. Thermoelastic experiments at room temperature, comprising simultaneous measurements of the mechanical work and of the concomitant heat, were performed on semicrystalline PAEKS. This study uses the sensitive thermoelastic technique to examine PAEK thermoelastic properties as a function of the relative abundance of ketone groups on different PAEKs. The effect of PAEK microstructure on thermoelasticity at room temperature was insignificant. This was attributed to the dominant contribution of the immobile amorphous phase on thermoelastic properties.