The effects of annealing semicrystalline polymers in the presence of plasticizing agents is an area of considerable current interest, given the potential to modify the degree and nature of crystallinity. These effects were studied for two semicrystalline polymers, custom-synthesized methyl-substituted poly(aryl ether ether ketone) (MePEEK) and industrial-grade poly(ethylene terephthalate) (PET). Small-angle X-ray scattering (SAXS) was used to characterize the microstructure of both amorphous and preannealed materials. Differential scanning calorimetry (DSC), wide-angle X-ray scattering, and density measurements were also performed for the PET samples, and reference is made to similar analysis work done for MePEEK. A distinct morphological effect could be identified from SAXS measurements of MePEEK annealed in a stepwise fashion in the presence of high-pressure CO2 with the polar cosolvent CH3OH. This result was absent in MePEEK similarly annealed in air and supports earlier DSC measurements. A very different morphological effect of pressure alone was observed in PET annealed in pure CO2 (170 and 510 atm) at a temperature of 150 degrees C, well above the glass transition.