Method of improving thermal stability

The invention relates to a method of improving the thermal stability of natural or synthetic polyamide fibre materials that are undyed, fluorescent whitened or dyed with reactive or disperse dyes, with the exception of metal complex dyes, wherein the fibre material is treated with a liquor comprising a compound of formula (1) Y1-X-Y2 (1) wherein X is a divalent aliphatic or cycloaliphatic radical and 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; Q-Ĉalkyl unsubstituted or substituted by one or more hydroxy, amino or carboxyl groups or halogen atoms; C5-C24-aryl unsubstituted or substituted by one or more hydroxy, amino or carboxyl groups or halogen atoms; C6-C36aralkyl unsubstituted or substituted by one or more hydroxy, amino or carboxyl groups or halogen atoms; or C5-C24cycloalkyl unsubstituted or substituted by one or more hydroxy, amino or carboxyl groups or halogen atoms.