Curing kinetics and thermal degradation of phosphate-modified thiodiphenol-containing epoxy copolymer (P3-ETP) were conducted. The kinetic study reveals that P3-ETP resin has better curing properties than unmodified epoxy resin (ETP). The activation energy, in kJ/mol, was 61.5 for P3-ETP, 89.1 for ETP, 109.9 for Epon 1001, and 124.7 for Epon 828, respectively. The low activation energy of P3-ETP may indicate that the penta-coordinated phosphate moieties facilitated the self-crosslinking process by providing the needed flexibility of the epoxy backbone. Thermal degradation study reveals that the char yield at 850 degrees C was 33.0% for P3-ETP, 24.8% for P3-EBPA, and 8.9% for E-1001 under nitrogen atmosphere. The thermal transformation of the phosphate moiety in P3-ETP into phosphate salt was confirmed by the presence of P and S elements in the char residue by scanning electronic microscope/energy dispersive X-ray and by the observation on the delaying emission of the thermal degradative volatiles in the Direct pyrolysis-gas chromatography/mass spectrometric analysis. The flame retardancy of the phosphate moieties with thiodiphenol (TDP) as a whole was found effective, judging from the char yield. This may imply that sulfur atom in TDP also providing additional thermal stability for the P3-ETP epoxy copolymer.