“Sulfhydration” has been proposed to be a means of hydrogen sulfide (H 2 S) signaling in which the H 2 S reacts with the sulfur of a cysteine residue in proteins, thereby forming a polysulfide and changing the function of the latter, as described by the equation RSH + H 2 S → RSSH. However, this requires oxidation of the cysteine RSH, which cannot be accomplished by H 2 S, as the sulfur is in its most reduced form (oxidation state of −2). Hydropersulfides (H 2 S n , n = 2–8, oxidation number = 0), can oxidize RSH and have been suggested to be a contaminant of salts commonly used to produce H 2 S (NaSH, Na 2 S) and account for much of the activity originally attributed to H 2 S. Here, we use the green fluorescent protein, roGFP2, that contains two reactive cysteine residues (Cys147, Cys204) and is a specific reporter of redox status, being fully oxidized by 30 mM tert-butyl hydroperoxide (t-BuOOH), and fully reduced by dithiothreitol (DTT, 30 mM), to determine if H 2 S can spontaneously form hydro-persulfides in normoxic (21% O 2 ) laboratory conditions. We show that all concentrations ranging from 1 μ M-300uM H 2 S have very little reductive effect on roGFP, but that H 2 S > 300 uM slowly reoxidizes roGFP in a closed system given an hour to react. We attribute this to spontaneous oxidation of H 2 S to hydropersulfides. Addition of mixed persulfides (K 2 S n , n = 1–8) oxidizes roGFP at low concentrations (1uM-300uM), supporting our hypothesis. These results show that H 2 S n can be formed from H 2 S and thereby signal via sulfhydration. Support, NSF IOS-1051627 and NSF DGE-1313583.