Hydrogen sulfide (H 2 S) is an important biological messenger, which recently joined nitric oxide (NO) and carbon monoxide (CO) as an endogenously produced gasotransmitter. This discovery resulted in a renewed interest towards H 2 S and, in particular, in its reactivity with biological and bioinorganic targets. However, in the scientific literature studies exploring the reactivity of H 2 S/HSwith non-heme proteins are rare. In this work, we studied the interaction of HSwith the natural metalloprotein Cu-azurin and its cobalt variant via UV-Vis, fluo- [a]