Derivatized poly(styrene-co-divinylbenzene) resins have been prepared from the commercial crosslinked polystyrene by the aromatic electrophilic substitution reaction using PCl3/AlCl3 or PCl5/AlCl3, followed by basepromoted hydrolysis. The unmodified copolymer and their phosphored derivatives have been characterized by thermogravimetric (TG) analysis to evaluate their thermal stability in nitrogen atmosphere. The influence of phosphorylation and phosphonation of crosslinked copolymer on its thermal behavior was evaluated. The TG and DTG curves of modified copolymers have clearly shown two thermal degradation stages. The modified copolymers have presented higher thermal stability than unmodified one, showing that the introduction of phosphinic or phosphonic acid groups modifies their kinetic pathways. The decomposition temperatures and thermal stability were dependent on the phosphorous amount incorporated on the commercial resin. The modified copolymers were also evaluated by spectrometric method (FTIR, using KBr pellet). The phosphorous content of the modified copolymers was determined by colorimetric method based on sodium molybdate reagent (determination at 830 nm).