The kinetics of comproportionation of hypothiocyanous acid (HOSCN) and thiocyanate (SCN-) to give thiocyanogen ((SCN)(2)) in acidic aqueous solutions have been determined by double-mixing stopped-flow UV spectroscopy. Hypothiocyanite (OSCN-) was generated at pH 13 by oxidation of excess SCN- with hypobromite (OBr-), followed by a pH jump to acidic conditions ([H+] = 0.20-0.46 M). The observed pseudo-first-order rate constants exhibit first-order dependencies on [H+] and [SCN-] with overall third-order kinetics. The corresponding kinetics of hydrolysis of (SCN)(2) have also been examined. Under conditions of high (and constant) [H+] and [SCN-], the kinetics exhibit second-order behavior with respect to [(SCN)(2)] and complex inverse dependences on [H+] and [SCN-]. Under conditions of low [H+] and [SCN-], the kinetics exhibit first-order behavior with respect to [(SCN)(2)] and independence with respect to [H+] and [SCN-]. We attribute this behavior to a shift in the rate-limiting step from disproportionation of HOSCN (second-order dependency on [(SCN)(2)]) to rate-limiting hydrolysis (first-order dependency on [(SCN)(2)]). Thus, we have determined the following equilibrium constant by the kinetic method: (SCN)(2) + H2O -><- HOSCN + SCN- + H+; K-hyd = [HOSCN][SCN-][H+]/[(SCN)(2)] = k(hyd)/k(comp) = 19.8(+/- 0.7) s(-1)/ 5.14(+/- 0.07) x 10(3) M-2 s(-1) = 3.9 x 10(-3) M-2.