Soaking crystals of rhodanese (thiosulphate : cyanide sulphurtransferase) in 2 M caesium sulphate reveals three caesium binding sites of this enzyme. One of these had been described before as a binding site for sodium ions and is located in a cleft close to the active site. In this site the monovalent cation is coordinated by five oxygen atoms. The first additional binding site seems to be quite special. The caesium ion is bound to the phenyl ring of a tryptophan residue. It is further liganded by two oxygen atoms. The third binding site is a result of crystal packing effects: caesium is liganded by four oxygen atoms, provided by two rhodanese molecules and one sulphate ion.It is likely that the binding of caesium affects the fluorescence of the tryptophan residue with which it interacts. Such possible effects should also be kept in mind when caesium ions are used as a quencher in fluorescence studies of proteins in general.