A simple and confirmative method for quantitative determination of carbon monoxide in tuna and mahi-mahi tissues using GC/MS, following chemical liberation of CO into headspace, is described. Carbon monoxide in recent years has been employed by the fishery industry to preserve fresh appearance in selected species of finfish during frozen storage, particularly in vacuum-packaged products. Indigenous CO contents of fresh Ahi tuna and mahi-mahi were examined using the method described in this study and found to be close to or less than 150 and 100 ng/g, respectively. Commercially CO-treated, vacuum-packaged tuna from multiple sources consistently showed CO level near or greater than 1 mug/g, while CO level in the only CO-treated frozen mahi-mahi sample was in the 500 ng/g range. The difference between untreated and treated specimens was in the range of 1 order of magnitude and thus suggested an easy quantitative and confirmative method of CO using widely available instrumentation that may be potentially useful for regulatory purpose in determining whether a commercially available product has been exposed to CO even if not labeled as such.