The stereoregularity of poly(methyl methacrylate) (PMMA) prepared by various methods was characterized by pyrolysis-gas chromatography/mass spectroscopy (Py-GC/MS) at 500 degrees C under He carrier gas. The diad tacticity determined from the relative peak intensities of the diastereomeric tetramers in the pyrograms was consistent with that obtained by H-1 NMR, suggesting that no appreciable thermal isomerization occurred during the pyrolysis. The thermal degradation mechanisms to yield the diastereomeric tetramers from PMMA without isomerization were discussed, with the possibility of estimating the triad tacticity of PMMA from the distribution of diastereomeric pentamers.