Fluorescence quenching rate constants of some aromatic hydrocarbons by Zn2+, Ag+, Cd2+, In3+, Sn2+, Cs+, Hg2+, Tl+, and Pb2+ were determined in aqueous and N,N-dimethylformamide solutions. Paramagnetic interactions, the heavy atom effect, and electron transfer were excluded as a possible quenching mechanism. N2 gas laser photolysis studies revealed that a cation radical of the fluorescer was detected only for N-ethylcarbazole quenched by Ag+, Pb2+, and Hg2+ in N,N-dimethylformamide and for 1-pyrenesulfonic acid by Hg2+ in water. All other systems yielded the triplet state of the fluorescer quantitatively. The intermediates observed in the microsecond time region are the transient species with the lowest free energy. Picosecond laser photolysis of the Ag+ and Pb2+ quencher systems in N,N-dimethylformamide confirmed directly that the triplet state is induced by fluorescence quenching. On the basis of these results, it has been concluded that fluorescence quenching is due to nonfluorescent complex formation followed by rapid intersystem crossing. The electronic and geometrical structures of this complex were considered and compared to the excited aromatic hydrocarbon-halogen anion systems.