The aqueous reaction of acidic Cl-2 with excess SCN- rapidly generates a UV-absorbing intermediate identified as an equilibrium mixture of thiocyanogen, (SCN)(2), and trithiocyanate, (SCN)(3)(-). The decomposition of this mixture can be described as 3(SCN)(2) + 4H(2)O --> 5HSCN + H2SO4 + HCN. Under our conditions the decomposition is sufficiently slow that its kinetics can be studied using standard stopped-flow methodology. Over the pH range 0-2 the decomposition rate law is -d[(SCN)(2)]/dt = (3/2){k(di)s(p)K(hyd) (2)[(SCN)(2)](2)/([SCN-](2)[H+](2) + K-(SCN)3 -[SCN-](3)[H+](2) + K-hyd[SCN-][H+])} with K(SCN)3- = 0.43 +/- 0.29 M-1, K-hyd = (5.66 +/-0.77) x 10(-4) M-2, and k(disp) = (6.86 +/- 0.95) x 10(4) M-1 s(-1) at 25 degreesC and mu = 1 M. The K-(SCN)3- and K-hyd terms are significant enhancements relative to one of the rate laws conventionally cited. In the proposed mechanism, K(SCN)(3)(-) refers to the formation of (SCN)(3)(-) by association of SCN- with (SCN)(2), K-hyd refers to the hydrolysis of (SCN)(2) to form HOSCN, and k(disp) is the rate constant for the bimolecular irreversible disproportionation of HOSCN, which leads ultimately to SO42- and HCN. Ab initio calculations support the values of K-(SCN)3- and K-hyd reported herein. The high value for k(disp) indicates that HOSCN is a short-lived transient, while the magnitude of Khyd provides information on its thermodynamic stability. These results bear on the physiological role of enzymes that catalyze the oxidation of SCN- such as salivary peroxidase and myeloperoxidase.