In aqueous media at 25 degreesC [Mo(CN)(8)](3-) is reduced by thioglycolic acid (HSCH2COOH, TGA), and the reaction is strongly accelerated by the presence of trace amounts of copper ions. Dipicolinic acid (dipic) is an effective inhibitor of the copper catalysis. Both with and without dipic the reaction has the stoichiometry 2[Mo(CN)(8)](3-) + 2TGA --> 2[Mo(CN)(8)](4-) + RSSR, where RSSR is the disulfide derived from formal oxidative dimerization of TGA. In the presence of dipic, PBN (N-tert-butyl-alpha-phenyl-nitrone), and with a large excess of TGA the rate law for consumption of [Mo(CN)(8)](3-) is first order in both [TGA] and [Mo(CN)(8)(3-)]. The complex pH dependence is consistent with -SCH2CO2- being highly reactive (k = 1.8 x 10(4) M-1 s(-1)), the monoanion being less reactive, and HSCH2CO2H being unreactive. A mechanism is proposed in which the dianion undergoes electron transfer to [Mo(CN)(8)](3-), thus generating the thiyl radical. Analysis of the electron-transfer rate constant in terms of Marcus theory yields an effective self-exchange rate constant for the thiolate/thiyl redox couple that is in reasonable agreement with the value derived previously from the reaction of TGA with [IrCl6](2-). When copper catalysis is inhibited, the two reactions differ substantially in that the yield of -O3SCH2CO2- is significant for [IrCl6](2-) but undetectable for [Mo(CN)(8)](3-).