Thermoplastic elastomers consisting of triblock copolymers (MBM) with outer syndiotactic poly(methyl methacrylate) (sPMMA) blocks associated to an inner polybutadiene (PBD) block have been modified by stereocomplexation with isotactic PMMA (iPMMA) at a constant 2/1 sPMMA/iPMMA (or s/i) mixing ratio. Toluene cast films of these stereocomplexes have been studied by thermal analysis as a function of molecular weight of the sPMMA blocks and the iPMMA homopolymer. Although the melting temperature of the stereocomplex (T-m) is independent of molecular weight of the sPMMA blocks ((M) over bar(n) sPMMA) in the studied range from 7000 to 46000, the melting enthalpy increases with increasing (M) over bar(n) sPMMA. This effect is, however, erased by the sample annealing at 140 degrees C for 15 h, and a melting enthalpy of ca. 33 J/g total PMMA is then observed whatever the copolymer composition. One melting endotherm is observed in case of low (M) over bar(n) sPMMA, although two melting endotherms are reported for higher (M) over bar(n) sPMMA depending, however, on the heating rate. Two endotherms are indeed observed at small (M) over bar(n) sPMMA as the heating rate is low. Molecular weight of iPMMA has no significant effect on either the melting temperature or the melting enthalpy of the stereocomplexes. Dynamic mechanical analysis has confirmed the phase separation of the complexed triblock copolymers. At constant molecular weight of PBD and iPMMA, stereocomplexation increases the tensile strength of MBM copolymers containing small sPMMA blocks, whereas the opposite effect is observed for longer sPMMA blocks. When the same MBM copolymer is complexed by iPMMA, the tensile properties are independent of the molecular weight of iPMMA at least in the range from 5000 to 74000.