Fluorescent sensor for Hg2+ ions based on a fluorescein derivative (FS) has been synthesized and characterized by a variety of methods. FS itself was non-fluorescent but upon the addition of Hg2+, it gave rise to strong fluorescence emission due to irreversible desulfurization reaction with the spirolactam ring-opening process, foIlowed by cyclization of a thiosemicarbazide to form 1,3,4-oxadiazole. Furthermore, the Hg2+-triggered fluorescence response was not affected by other metal ions such as Ag+, Pb2+, Cd2+, Cr-3+, Zn2+, Fe2+, Co3+, Ni +, Ca2+, Mg2+, K+ and Na+, indicating that FS could selectively detect Hg2+ ions.