High temperature pyrolysis studies of poly(phenylene vinylene)s PPVs with lateral substituents poly(epsilon-caprolactone) (PPV-PCL) or poly(epsilon-caprolactone) and alternating Br (PPV-PCL-Br) or polystyrene (PPV-PSt) clearly showed that thermal stability of both the substituent and PPV were affected by the thermal stability of the other. In all the polymers under investigation, decomposition started by the degradation of the substituent. The thermal stability of the PPV backbone increased in the order PPV-PCL-Br < PPV-PCL < PPV-PSt. When the thermal stability of the substituent was significantly lower than that of the PPV backbone, as in the case of PPV-PCL and PPV-PCL-Br, then the radicals generated at early stages of pyrolysis coupled before the temperature reached to the values necessary for complete decomposition. This inturn yielded a thermally more stable crosslinked structure. The increase in thermal stability was greater upon coupling of the radicals generated on the PPV backbone.