The one-electron reduction of triiodide (I-3(-)) by a reduced ruthenium polypyridyl compound was studied in an acetonitrile solution with the flash-quench technique. Reductive quenching of the metal-to-ligand charge-transfer excited state of [Ru-II(deeb)(3)](2+) by iodide generated the reduced ruthenium compound [Ru-II(deeb(-))(deeb)(2)](+) and diiodide (I-2(center dot-)). The subsequent reaction of [Ru-II(deeb(-))(deeb)(2)](+) with I-3(-) indicated that I-2(center dot-) was a product that appeared with a second-order rate constant of (5.1 +/- 0.2) x 10(9) M-1 s(-1). After correction for diffusion and some assumptions, Marcus theory predicted a formal potential of -0.58 V (vs SCE) for the one-electron reduction of I-3(-). The relevance of this reaction to solar energy conversion is discussed.