Steady-state and time-resolved absorption and fluorescence measurements were used to probe the mechanism(s) of excited singlet state quenching by stable nitroxyl radicals in a series of fluorescamine-derivatized nitroxides. Rate constants for intramolecular quenching (k(q)) acquired from fluorescence lifetime measurements were very high, ranging from similar to 0.3 x 10(10) s(-1) to greater than or equal to 5 x 10(10) s(-1), and showed little dependence on solvent polarity. The k(q)＇s did not track the values of the Dexter and Forster spectral overlap integrals, thus indicating that energy transfer, through either mechanism, cannot account for the quenching. Quenching by electron transfer also appears unlikely, owing to the dependence of k(q) on solvent polarity and the complete lack of radical ion generation in highly polar solvents. Time-resolved absorption measurements revealed the presence of a very weak transient centered at similar to 470 nm in the diamagnetic reference compounds that was not observed in the paramagnetic nitroxides; this transient was tentatively assigned to the excited triplet state of the fluorescamine moiety. The rapid singlet state quenching in this series of compounds thus appears to arise from enhanced internal conversion.