Hydrogen sulfide (H2S) is an important gaseous mediator, like nitric oxide, that has significant effects on the immunological, neurological, cardiovascular and pulmonary systems of mammals. WSP-5 is a reactive disulfide-containing, fluorescent probe designed for fast detection of H2S in biological samples and cells. Through a reaction-based fluorescent turn-on strategy, WSP-5 selectively and rapidly reacts with H2S, generating a fluorophore with excitation and emission maxima of 502 and 525 nm, respectively. WSP-5 demonstrates a faster fluorescence turn-on rate (60- vs. 130-fold) and a more sensitive detection limit (47 vs. 60 nM) toward H2S compared to WSP-1 .