The effect of organic phosphates such as inositol hexaphosphate (IHP), ATP, and 2,3-diphosphoglycerate (2,3-DPG) on hemoglobin oxidation by hydroxylamine was studied under aerobic and anaerobic conditions. Under aerobic conditions, the oxidation rate of hemoglobin by hydroxylamine was accelerated as much as about 1.6 times, 1.4 times, and 1.4 times in the presence of IHP, ATP, and 2,3-DPG, respectively; however, under anaerobic conditions it was inhibited as much as 2.2 times, 2 times, and 2 times in the presence of these organic phosphates compared with the case for the absence of these organic phosphates. The effects of these organic phosphates on hemoglobin oxidation by hydroxylamine under aerobic conditions were elucidated by the two state models of hemoglobin including the R and the T state, however, the opposite effects under anaerobic conditions cannot be explained without the assumption of the existence of a third conformation of hemoglobin. The dissociation constant of IHP to deoxyhemoglobin was estimated from the rate of the hemoglobin oxidation by hydroxylamine in different concentrations of IHP under anaerobic conditions.