Second excited singlet state (S2) fluorescence quantum yields and lifetimes of six aromatic thiones have been measured in solution at room temperature. Intramolecular S2 S, internal conversion dominates S2 decay in inert perfluoroalkane solvents, but intermolecular photochemical and photophysical processes dominate in more strongly interacting solvents. Stern-Volmer quenching is observed when perfluoroalkane solutions of 2,2,3,3-tetramethylindanthione, a model thione, are spiked with addends at concentrations up to ca. 0.3 M. Measurements of the rate constants for the bimolecular quenching of S2 fluorescence by 26 different addends reveal that the thione S2 state is highly reactive and that the initial intermolecular interaction path divides into physical and chemical branches, the branching ratio being a function of the nature of the quencher.