The very rapid nitric oxide dioxygenation (NOD) reaction of nitric oxide (NO) with the oxygen bound to the ferrous derivatives of hemeproteins such as hemoglobin and myoglobin to yield nitrate and the ferric derivate (met) of the hemeprotein is of considerable physiological and biomedical importance. The mechanism for this reaction has been elusive due to the rapidity of the reaction. This article describes a method based both on using trehalose-derived glassy matrices to control the reaction of NO with oxyhemoglobin through both a temperature and glass-dependent modulation of the protein dynamics and a novel method of generating NO within the glassy matrix. The results support models in which there is a very rapid formation of an intermediate that immediately decays into an initial nonequilibrium population of high and low ferric nitrate that on a slower time scale relaxed to an easily dissociated equilibrium form of the ferric nitrate derivative of hemoglobin.