When nitrosothiols react with excess hydrogen sulfide, H2S, they form several intermediates including nitrosopersulfide (SSNO−). The stability and importance of this species has been debated. While some data suggest SSNO− can be a relatively stable source of NO activity, others suggest that the species degrades too quickly. We find the species to be relatively stable in isolation. Due to the abundance and prominence of iron-containing proteins throughout the human body, it is important to establish the interaction of ferrous- and ferric-iron containing proteins with SSNO-. Study of the reactions of SSNO− with heme proteins can also provide information about the potential in vivo stability and spontaneous reactivity of this species. We have used time-resolved electron paramagnetic resonance and UV–Vis absorption spectroscopy to study the reactions of SSNO− with heme proteins. Iron-nitrosyl hemoglobin is formed when SSNO− is reacted with deoxyhemoglobin and deoxygenated methemoglobin, suggesting NO formation from SSNO−. However, the yields of nitrosyl hemoglobin in reactions of SSNO− with deoxyhemoglobin are much less than when SSNO− is reacted with deoxygenated methemoglobin. Very little to no nitrosyl hemoglobin is formed when SSNO− is reacted carboxyhemoglobin, HbCO, and when SSNO− is reacted with oxygenated hemoglobin, minimal methemoglobin is formed Taken together, these data confirm the release of NO, but indicate a vacant heme is necessary to facilitate a direct heme-SSNO− reaction to form substantial NO. These data also suggest that the ferric iron in methemoglobin potentiates SSNO- reactivity. These results could potentially impact NO and sulfide bioavailability and reactivity.