syn-1,5-Diazabicyclo[3.3.0]octadienediones (syn-9,10-dioxabimanes, syn-(R2,R1)B) are usually strongly fluorescent. Donor moieties in one or both R2 groups quench the fluorescence in a manner dependent on the donor strength and solvent polarity. Charge transfer is strongly implicated as the quenching mechanism; no charge-transfer emission is observed. Solvent effects on the fluorescence maxima for all compounds are parallel, but the fluorescence quantum yields (φF) for the compounds with donor substituents fall dramatically from 0.7 to values between 0.001 and 0.01 over narrow solvent polarity ranges. The rate constant for electron transfer (quenching), ket, is estimated from the fluorescence quantum yields and radiative rates, with the substituent effect very well correlated by the Brown σ+ substituent constant (ρ value, -6, hydrocarbon solvent). Reduction potentials are used to evaluate bimane acceptor strengths. Fluorescence quenching requires conformational mobility within the molecule and/or in the solvent as shown by the increase in fluorescence observed in glasses or viscous solvents. Added amines quench the fluorescence at diffusion-controlled rates. A substantial difference between the two pKa values [Δ = 5.5] for the bisprotonated, bis(piperidino) derivative, syn-(C5H10NH+CH2,CH3)B, is due to the stability of an intramolecularly hydrogen-bonded monoprotonated form, for which the rather low φF (ca. 0.01) implies a structure.