Process for preparing salts of the formula I (B+)nxAy− where B is a cation comprising at least one nitrogen atom, A is an anion and n is an integer from 1 to 3, x and y are each an integer from 1 to 3 and the product of x and y is equal to n, by reacting salts of the formula II (B+)nxCy− where B and n, x and y are as defined above and C is a compound which has one or more carboxylate groups (referred to as carboxylate for short) and is different from A, with the ammonium salt of the anion A or with the protic acid of the anion A in the presence of ammonia.