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Fluorescence decays of benzosultine and naphthosultine excited at 263 nm are detected with time-correlated single-photon counting (TCSPC). Both molecules display biexponential decay with a rapid component (time constants 90 and 350 ps for benzosultine and naphthosultine, respectively) assigned to an electronically excited state S n at high energy and the slow one (constants 7.5 and 9.0 ns for benzosultine and naphthosultine, respectively) to the S 1 state. Dissociation of both molecules was investigated with nanosecond laser flash photolysis combined with transient absorption to detect the intermediates and products. With excitation at 266 nm, benzosultine yields a transient with absorption maximum at λ max = 370 nm; this transient has a short-lived component with lifetime around 1 μs and a long-lived component. Both components are insensitive to molecular oxygen. The short-lived component is tentatively assigned to the dissociation intermediate and the long-lived to be singlet o -benzoquinodimethane ( o -BQDM). Photolysis of naphthosultine yields two transient species with absorption at λ max = 420 and 520 nm; we assign the former band to triplet–triplet absorption of naphthosultine and the latter to absorption by the product singlet o -naphthoquinodimethane. Optimal geometries, energetics, and vertical transitions of benzosultine, naphthosultine, o -benzoquinodimethane, and o -naphthoquinodimethane are calculated using methods based on density-functional theory (B3LYP/6-311++G**) and time-dependent density-functional theory (TD-DFT). The results of these calculations imply that the ground electronic state of these two o -quinodimethanes ( o -QDM) is singlet with a structure of diene form. Their triplet states display a biradical structure. The energy separation between singlet and triplet states of o -benzoquinodimethane is calculated to be ca. 93.6 kJ/mol, but for o -naphthoquinodimethane, it is only ca. 27.8 kJ/mol.