The thermal degradations of thiophene-capped poly(methyl methacrylate) and poly(methylthienyl methacrylate) were studied via direct pyrolysis mass spectrometry. No significant effect of heating rate on the thermal degradation behaviors of the polymers under investigation have been observed in the heating range studied. It has been determined that thermal degradation of thiophene-capped poly(methyl methacrylate), (TPMMA) mainly yields monomer as in the case of pristine poly(methyl methacrylate). An analogous degradation mechanism can also be proposed for poly(methylthienyl methacrylate), (PMTM). Yet, the elimination of side chains, evolution of OCH2C4H3S groups was found to be also effective for this sample. Pyrolysis experiments were repeated using 17 eV electrons to determine the extent of fragmentation due to dissociative ionization.