Thermofixation of the reactive disperse azo dye, 2-hydroxy-5-methyl-4'-sulphonazidoazobenzene ( I ) containing a sulphonazido group, with polyester fibre leads to chemical bonding of the generated sulphonylnitrene with the fibre substrate. The nitrene interacts with the benzene ring of the polyester substrate through the formation of a sulphonamido link as well as through insertion followed by ring expansion of the adduct to yield the relevant N-sulphonyl-azepine. The mechanism is confirmed by means of a suitable model experiment of interaction of I with dimethyl terephthalate.