This part of ISO 3451 specifies methods for determination of the ash of poly(ethylene terephthalate), poly(butylene terephthalate) and copolymers of the two, both filled and unfilled. The general procedures given in ISO 3451-1 are followed. For unfilled materials method A or method C of ISO 3451-1:1997 is used.

For filled and glass-fibre reinforced materials method A of ISO 3451-1:1997 is used. For glass-fibre filled materials, containing flame retardant, antimony trioxide, and/or other, volatilize additives, for instance pigment zinc sulfide, a modification is incorporated to remove these as volatile bromine component(s).