Crystalline stereocomplexes of highly stereoregular isotactic poly(methyl methacrylate) (it-PMMA; mm = 97.2%) and syndiotactic poly(methyl methacrylate) (st-PMMA; rr < 86.1%) blends formed. by treatment with supercritical CO2 at pressures from 5 to 25 MPa and at temperatures in the 30-170 C range were investigated by means of differential scanning calorimetry (DSC) measurements as a function of treatment temperature, pressure, and it-PMMA/st-PMMA mixing ratio. The DeltaH of the stereocomplex had a maximum at 33 wt % it-PMMA content, in agreement with stereocomplexes formed in some solvents and in bulk thermal annealing methods. For the case of over 50% it-PMMA content, two endothermic peaks, T-m(1) and T-m(3), were detected. Examination of the effect of heating rate from 5 to 40 degreesC/min on the DSC results showed that the melting temperatures, T-m(1) and T-m(3), corresponded to the fringed micellar structure and the lamellar crystallites of the complexed sections, respectively. For 33% or less it-PMMA content, the crystalline stereocomplex formed by supercritical CO2 had only one endothermic peak that occurred over a narrow temperature range, which was comparable with that obtained by thermal annealing or solvent treatment.