Reduction of [AuCl4]- and trans-[Au(CN)2X2]- (X = Cl, Br) by sulfur(IV) as SO2.nH2O, HSO3-, and SO3(2-) has been studied at 25-degrees-C in aqueous solution with ionic strength 1.0 M and 0 < pH < 2.3 by use of stopped-flow spectrophotometry. Redox takes place directly without initial substitution at the gold(III) centers with stoichiometry Au(III):S(IV) = 1:1 and with Au(I) complexes and HSO4- as products. A mechanism with two parallel redox reactions and HSO3- and SO3(2-) as reductants results in the following respective rate constants for reduction of [AuCl4]-, trans-[Au(CN)2Cl2]-, and trans-[Au(CN)2Br2]- : by HSO3-, 35 +/- 9, (1.5 +/- 0.2) x 10(2), and (1.7 +/- 0.2) x 10(3) M-1 s-1; by SO3(2-) (6.8 +/- 0.4) x 10(6), (1.6 +/- 0.1) x 10(7), and (1.8 +/- 0.1) x 10(8) M-1 s-1. Reduction is ca. 10(5) times faster with SO3(2-) than with HSO3-. A halide-bridged, two-electron transfer in a transition state where the sulfur of the reductant interacts with the halid ligand and which is further stabilized through direct interaction between the positive metal center and the negatively charged oxygen of the sulfite/hydrogen sulfite is proposed. Reduction of trans-[Au(CN)2Br2]- is ca. 10 times faster than reduction of trans-[Au(CN)2Cl2]-, in agreement with bromide being a more efficient bridging ligand for electron transfer. In the case of [AuCl4]- and trans-[Au(CN)2Cl2]-, there is also a parallel solvolytic pathway, with the acid hydrolysis of these complexes being rate-determining for the reduction, with rate constants (2.4 +/- 0.6) x 10(-2) and (5.6 +/- 1.4) x 10(-2) s-1, respectively. Intermediate formation of sulfite radicals and formation of dithionate as reaction product in the [AuCl4]- reaction, as claimed in recent literature, can most likely be ruled out.