Frémy's salt is a chemical compound with the formula (K4[ON(SO3)2]2), sometimes written as (K2[NO(SO3)2]).

[3] Regardless of the cations, the salts are distinctive because aqueous solutions contain the radical [ON(SO3)2]2−.

[4][5][6] The inorganic aminoxyl group is a persistent radical, akin to TEMPO.

[12][13] It can also be used as a model for peroxyl radicals in studies that examine the antioxidant mechanism of action in a wide range of natural products.

Oxidation is typically conducted at low-temperature, either chemically or by electrolysis.