A series of thermally crosslinkable polyester copolymers were synthesized by incorporation of a benzocyclobutene-containing terephthalic acid derivative (XTA) into polyethylene terephthalate (PET). The cyclobutene moiety on the XTA monomer allows for reactive crosslinking at temperatures similar to 350 degrees C requiring no catalyst and causing no change in mass. Copolymers were synthesized containing 1, 5, 10, 20, 50, and 100 mol% XTA. Crosslinking occurred above the melting temperature (similar to 250 degrees C) yet below the degradation temperature (similar to 400 degrees C), providing a window for melt processing of the copolymer. To demonstrate this point fibres were melt spun. The PET-co-XTA copolymers show systematic variations in the glass transition, recrystallization, melting and degradation temperatures as a function of benzocyclobutene content. The degradation and melting temperature both decrease slightly with increased XTA, while the recrystallization and glass transition temperature were relatively insensitive to XTA content. Thermal gravimetric analysis (TGA) indicated a decrease in the degradation temperature as higher amounts of XTA were incorporated, although an increase in the %char at 800 degrees C was seen. This decrease in degradation temperature may be due to the generation of free radicals. Limiting Oxygen Index (LOI) measurements showed an increase in the oxygen content required to maintain a stable flame in copolymers with increasing amounts of XTA. LOI values ranged from 18 for neat PET to 35 for the copolymer containing 20 mol % XTA. Wide-angle X-ray scattering data showed little change in the crystalline structure, but decreasing crystallinity for PET for blends containing up to 20 mol% XTA. The 50 mol% XTA copolymer was amorphous, while the 100% XTA homopolymer (PEXTA) showed evidence of a new crystalline structure. Crystalline diffraction peaks showed reduced intensities in data recorded for heat treated samples, and there was evidence for new peaks in the copolymer containing 20 mol% XTA when heated near 300 degrees C. Transmission electron microscopy of cross-sections through burned samples showed a highly crystalline char at the surface of XTA copolyesters. This crystalline char appeared to protect the underlying copolymer from further flame-induced degradation. Evidence for significantly increased adhesion of the copolymers to polyimide films was also obtained.