Frémys salt is a chemical compound with the formula (K[ON(SO)]), sometimes written as (K[NO(SO)]). It is a bright yellowish-brown solid, but its aqueous solutions are bright violet. The related sodium salt, disodium nitrosodisulfonate (NDS, NaON(SO), CAS 29554-37-8) is also referred to as Frémys salt.
Regardless of the cations, the salts are distinctive because aqueous solutions contain the radical [ON(SO)].