The glass transition temperature of MePEEK, a monosubstituted methyl derivative of poly(aryl ether ether ketone), PEEK, is 145-degrees-C, the same as that of PEEK. However, the onset of melting of MePEEK, crystallized by thermal annealing, occurs at 210-degrees-C, much lower than the onset at 335-degrees-C observed for PEEK. MePEEK, crystallized by successively annealing for 100 min each at 175, 190, and 200-degrees-C, melts in a two-step process spread over a broad temperature range of about 50 deg. This double melting behavior is observed in PEEK also. When MePEEK is annealed in a similar manner in the presence of supercritical CO2 at 100 atm, the melting event is still broad but the two peaks are now overlapped. Similar conditioning in the presence of supercritical 0.9CO2 + 0.1CH2Cl2, and 0.9CO2 + 0.1CH3OH at 100 atm gives a single and sharp melting peak with onset at 229 and 260-degrees-C, respectively. The melting event is now spread over about 15 deg only. The density of amorphous MePEEK is 1.233 g cm-3. The density and heat of melting of 100% crystalline MePEEK are estimated to be 1.348 g cm-3 and 74 J g-1, respectively, on the basis of density, thermal, and X-ray diffraction measurements on semicrystalline samples. The corresponding values for PEEK are 1.40 g cm-3 and 130 J g-1.