Formation kinetics of the metal-metal bonded [(CN)5PtTl(CN) 3]3- complex from Pt(CN)42- and Tl(CN)4- has been studied in the pH range of 5-10, using standard mix-and-measure spectrophotometric technique at pH 5-8 and stopped-flow method at pH > 8. The overall order of the reaction, Pt(CN) 42- + Tl(CN)4- ? [(CN) 5PtTl(CN)3]3-, is 2 in the slightly acidic region and 3 in the alkaline region, which means first order for the two reactants in both cases and also for CN- at high pH. The two-term rate law corresponds to two different pathways via the Tl(CN)3 and Tl(CN)4- complexes in acidic and alkaline solution, respectively. The two complexes are in fast equilibrium, and their actual concentration ratio is controlled by the concentration of free cyanide ion. The following expression was derived for the pseudo-first-order rate constant of the overall reaction: kobs = (k1a[Tl(CN) 4- + Kp[H+]))[CN-] free + k1b[TI(CN)4-] + (k1b/Kf), where k1a and k1b are the forward rate constants for the alkaline and slightly acidic paths, Kf is the stability constant of [(CN) 5PtTl(CN)3]3-, and Kp is the protonation constant of cyanide ion. k1a = 143 ± 13 M-2 s-1, k1b = 0.056 ± 0.004 M-1 s-1, Kf = 250 ± 54 M -1, and log Kp = 9.15 ± 0.05 (/ = 1 M NaClO 4, T = 298 K). Two possible mechanisms were postulated for the overall reaction in both pH regions, which include a metal-metal bond formation step and the coordination of the axial cyanide ion to the platinum center. The alternative mechanisms are different in the sequence of these steps.