Abstract: comonomers have been shown to improve flammability characteristics of polyesters and copolyesters. Incorporation of sulfonate containing monomers such as 5-sodiosulfoisophthalic acid significantly decreases the heat release capacity of copolyesters. These comonomers also increase the formation of char as measured by thermogravimetric analysis. The decrease in the heat release capacity and increase in char formation indicates that these compositions may have reduced flammability and thus are useful for producing products have reduced flammability. In particular, it has also been demonstrated that incorporation of as little as 0.1 mole % 5-sodiosulfoisophthalic acid to a polyester or copolyester significantly increases the char formation independent of composition.