Understanding of crystal growth is essential to the design of materials with improved properties. Unfortunately, still very little is understood about the basic growth mechanisms of nanostructures, even in the most established colloidal synthetic routes. Etching is one of the most important mechanisms to consider during particle growth, but it is rarely considered in the syntheses of oxide or chalcogenide nanostructures. Here, we report that the most common precursor for the synthesis of sulfide nanostructures – the mixture of sulfur and oleylamine – acts as a very powerful etchant for oxide, chalcogenide, and metal nanostructures. Specifically, we discuss its effect on several nanoparticle compositions (PbS, Cu2S, Fe3O4, and Au) and compare it to control conditions in which only oleylamine is present. Our experiments suggest that the etching results from the evolution of H2S from the sulfur–oleylamine precursor. We predict that the simultaneous role of this precursor as both etchant and ligand stabilizer will make it a useful tool for the chemical post‐processing (e.g., size reduction, focusing of size distributions, faceting) of nanocrystal dispersions.