Abstract: Blood samples are stabilized for methemoglobin determination with a buffer composition containing (a) carbon monoxide-containing water; (b) sodium tetraborate or potassium tetraborate; and (c) KCN or NaCN. The buffer composition preferably may also have an erythrocytolysis agent. The buffer fixes the valence state of heme iron at a concentration representative of the blood at the time of collection, and maintains that fixed state for an extended period of time preventing further methemoglobin formation. The buffer also prevents reduction of existing target analyte, i.e., methemoglobin (ferric hemoglobin, Fe3+ hemoglobin) to normal reduced hemoglobin (ferrous hemoglobin, Fe2+ hemoglobin).