The photoreduction of 5,7-diiodo-3-butoxy-6-fluorone (DIBF) in the presence of the electron donors, tetrabutylammonium triphenylbutylborate (BORATE), and N,N-dimethyl-2,6-diisopropylaniline (DIDMA) was studied in acetonitrile solution. The transient phenomena examined using nanosecond laser flash photolysis confirm that the reduction is a one-electron transfer process from the electron donor to the excited triplet state of the dye. Triplet quenching rate constants were measured for BORATE and DIDMA as well as oxygen. The relative quenching rates for oxygen and DIDMA are approximate to 20:1. The electron transfer rates calculated are 6.7 x 10(7) and 1.2 x 10(8) s(-1) in the BORATE and DIDMA systems, respectively. The dye radical anion (DIBF, lambda(max)approximate to 420 nm) formed by the electron transfer reaction is the only product obtained in the BORATE case. However, with DIDMA the dye "neutral" radical (DIBF,;lambda(max) approximate to 360 nm) was detected as the only product. DIBF. formed after protonation of DIBF.- by proton transfer from the amino cation radical. This implies that the proton transfer is too fast to measure on the nanosecond time scale. This is the first such observation in these systems. Singlet oxygen formed by oxygen quenching of the dye tripler was found to be quenched by the electron donors via an electron transfer process also. No quenching of DIBF triplet occurred in the presence of the electron acceptor (4-octyloxyphenyl)phenyliodonium hexafluoroantimonate (OPPI). The principal photobleached products detected by HPLC were reduced quinomethines in the DIDMA case and the ammonium salts of the reduced quinomethines in the BORATE case. Reoxygenation of these products regenerates the original dye in a reaction the rate of which depends on the polarity of the solution. However, the reoxygenation of the quinomethines is faster than that of their ammonium salts under the same conditions.