Trimethylamine N-oxide (TMAO) is the organic compound in the class of amine oxides with the formula (CH₃)₃NO. This colorless solid is usually encountered as the dihydrate. It is a product of the oxidation of trimethylamine and a common metabolite in animals. It is an osmolyte found in saltwater fish, sharks, rays, molluscs, and crustaceans. It is a protein stabilizer that may serve to counteract urea, the major osmolyte of sharks, skates and rays. It is also higher in deep-sea fishes and crustaceans, where it may counteract the protein-destabilizing effects of pressure. TMAO decomposes to trimethylamine (TMA), which is the main odorant that is characteristic of degrading seafood.