A process for the preparation of a bis(sulphonato)imide salt of formula: (III) (SO3−)—N−—(SO3−) 3C+ where C+ represents a monovalent cation, comprising the reaction of amidosulphuric acid of formula: (I) (OH)—SO2—NH2 with a halosulphonic acid of formula: (II) (OH)—SO2—X where X represents a halogen atom, and comprising a reaction with a base which is a salt formed with the cation C. Also, a process for the preparation of bis(fluorosulphonyl)imide acid of formula: (V) F—(SO2)—NH—(SO2)—F and to a process for the preparation of lithium bis(fluorosulphonyl)imide salt of formula: (VII) F—(SO2)—N−−(SO2)—F Li+.