The influence of thermal history on the stability of stereocomplexes of isotactic and syndiotactic poly(methyl methacrylate) PMMA was investigated using differential scanning calorimetry. It was found that a prolonged annealing of the stereocomplex not only increases its crystallinity but also leads to the appearance of another, yet rather small, endotherm with a maximum located close to the beginning of the melting of the stereocomplex. Crystallization from the melt and partial melting of samples indicate that the isotactic PMMA also is able to crystallize the stereocomplex crystals, the ratio of these two crystalline species being very sensitive to the stoichiometric ratio of the mixture and the isotactic content of the PMMA.