Treating seafood with carbon monoxide (CO) and various filtered wood smokes (FS) containing carbon monoxide has become a common practice in the industry, particularly for seafood intended for the frozen market. Tuna is one of the most common fish species treated with this process. Various quality improvement claims have been made for these treatments, which require investigation. Yellowfin tuna steaks were subjected to various CO treatments (4% CO, 18% CO and 100% CO) and FS treatment (which contained 18% CO) for 48 h, followed by 30 days of freezing and subsequent cold storage (4ЊC) after thawing. Muscle quality was evaluated after treatment. The FS, 18% CO and 100% CO treatments led to reductions in formation of secondary lipid