A Hg(2+) chemodosimeter P3 derived from a perylenebisimide scaffold and thiourea fragments was systematically studied with focus on the photophysical, chemodosimetric mechanistic, as well as fluorogenic behaviors toward various metal cations for the sake of improving selectivity to Hg(2+). As demonstrated, Hg(2+) can promote a stepwise desulfurization and N-acylation of P3 with the help of an acetate anion (OAc(-)), resulting in an N-acylated urea derivative. Interestingly, OAc(-) has the effect of improving the selectivity of P3 to Hg(2+) among other metal ions; that is, in an acetone/Britton-Robinson buffer (9:1, v/v; pH 7.0) upon excitation at 540 nm, the relative fluorescence intensity is increased linearly with increasing concentration of Hg(2+) in the range of 2.5-20 μM with a detection limit of 0.6 μM, whereas the fluorescence intensity of P3 to other metal ions, including Na(+), K(+), Mg(2+), Ca(2+), Mn(2+), Fe(2+), Ni(2+), Co(2+), Zn(2+) Ag(+), Cd(2+), Pb(2+), and Cu(2+), is negligible. The fluorescent bioimaging of chemodosimeter P3 to detect Hg(2+) in living cells was also reported.