The aqueous oxidation of thioglycolic acid (TGA) by [Os(phen(3))(3+) (phen = 1,10-phenanthroline) is catalyzed by traces of ubiquitous Cu2+ and inhibited by the product [Os(phen)(3)](2+). In the presence of clipicolinic acid (dipic), which thoroughly masks trace Cu2+ catalysis, and spin trap PBN, the kinetics under anaerobic conditions have been studied in the pH range 1.82-7.32. The rate law is -d[Os(phen)(3)(3+)]/dt = K[TGA](tot))[Os(phen)3(3+)], with k = 2{(k(b)Ka(1) + k(c)K(a),K-i)[H+] + k(d)K(a1)K(a2)}/{[H+](2) + K-a1[H+] + Ka(1)Ka(2)}; K-a1 and K-a2 are the successive acid dissociation constants of TGA, and K., is the tautomerization constant of two TGA monoanions. k(b) + k(c)K(i) = (5.9 +/- 0.3) X 103 M-1 s(-1), k(d) = (1.6 +/- 0.1) x 10(9) M-1 s(-1) at mu = 0.1 M (NaCF3SO3) and 25 degrees C. The major products in the absence of spin traps are dithiodiglycolic acid, [Os(phen)(3)](2+), and [Os(phen)(2)(phen-tga)](2+), where phen-tga is phenanthroline with a TGA substituent. A mechanism is proposed in which neutral TGA is unreactive, the (minor) thiolate form of the TGA monoanion undergoes one-electron oxidation by [Os(phen)(3)](2+) + (k(c)), and the dianion of TGA likewise undergoes one-electron oxidation by [Os(phen)(3)](3+) (k(d)). The Marcus cross relationship provides a good account for the magnitude of kd in this and related reactions of TGA. [Os(phen)(2)(phen-tga)](2+) is suggested to arise from a post-rate-limiting step involving attack of the TGA* radical on [Os(phen)(3)](3+).