PMMA stereocomplex formation was demonstrated in domains of PMMA-PDMS-PMMA block copolymers having syndiotactic (S) PMMA blocks by mixing a THF solution of the block copolymer and that of isotactic (I) PMMA in the S/I PMMA ratio of 2 followed by precipitation fi om THF or by forming the same blend by precipitation from chloroform followed by annealing at temperatures in the 100-140 degrees C range. The various blends and the block copolymer itself were analyzed by DSC and DMTA. The dissociation-melting temperature of the stereocomplexes present in the PMMA domains was shown to increase with increasing annealing temperature. The rate of formation of stereocomplexes in bulk was found to be much slower than that in blends of PMMA homopolymers. Crystallization of PDMS was shown to be hindered by stereocomplex formation. The modulus at temperatures above 100 degrees C of the blends in which stereocomplex formation occurred was found to be higher than in the same blends in which such a stereocomplex was absent. These results are interpreted by the formation of networks formed by fringed micellar crystallites of PMMA stereocomplexes acting as physical cross-links.