Nitric oxide (NO) plays a major role in human physiology and in many pathological states. Although oxyhemoglobin is known to destroy NO activity, NO activity can, in principle, be conserved through iron nitrosylation at vacant hemes. In order for this NO activity to be delivered, the NO must dissociate from the heme. Despite its study over the past few decades, our understanding of NO dissociation from hemoglobin is incomplete. In principle, there are at least four NO dissociation rates: k_R^α, k_R^β, k_T^α, and k_T^β, where the subscript refers to the quaternary state and the superscript to the hemoglobin chain. In the T-state, a proportion of the proximal histidine bonds break forming pentacoordinate α-nitrosyl hemoglobin. In vivo, α -nitrosyl hemoglobin predominates over β-nitrosyl hemoglobin. In this study we have used a fast NO trap, Fe(II)-proline-dithiocarbamate, to measure NO dissociation rates from hemoglobin. We have varied solution conditions so the rate of dissociation from pentacoordinate α -nitrosyl hemoglobin could be definitively measured for the first time; k_T^α = 4.2 ± 1.5 × 10⁻⁴ s⁻¹. We have also found that the fastest NO dissociation rate is on the order of 10⁻³ s⁻¹ and that NO dissociation from sickle cell hemoglobin is the same as that from normal adult hemoglobin.