The invention relates to an auxiliary composition comprising (A) from 5 to 95% by weight, based on the total composition, of at least one compound of formula (1a) or (1b) Y1—X—Y2  (1a) A1A2N—OH  (1b) wherein X is a divalent aliphatic, aromatic, araliphatic or cycloaliphatic radical, Y1 and Y2 are each independently of the other —OH, —CO—OR1, —NR1R2, —CO—NH—NR1R2 or —NH—CO—NH—NR1R2, wherein R1 and R2 are each independently of the other hydrogen, C1-C20alkyl, C1-C20alkoxy, C5-C24cycloalkyl, C5-C30aryl or C6-C36aralkyl, it being possible for the alkyl, alkoxy, cycloalkyl, aryl or aralkyl groups to be unsubstituted or substituted by one or more hydroxy, amino, sulfo or carboxyl groups or halogen atoms, A1 and A2 are each independently of the other C1-C20alkyl or C6-C36aralkyl; (B) from 0 to 95% by weight, based on the total composition, of one or more anionic or non-ionic surfactants or dispersants; and (C) from 0 to 85% by weight, based on the total composition, of a solid, inorganic or organic acid; the sum of the amounts of components A+B+C being 100% by weight in each case, and also to a method of improving the thermal stability of natural or synthetic textile fibre materials that are undyed, fluorescent whitened or dyed.